Thread: Freebore Decision for a .284 Winchester Reamer

Foreword

Summer, and Puffins such as myself are in their burrows. Burrow time means an opportunity to get in a bit of reloading, plan future builds and projects, and give some consideration to the more technical aspect of shooting.

In the past few days I have dug out an article penned some time ago, a reference piece, that I had reservations about posting at all. It is not that it is controversial or going to get me removed from the forum; but it is lengthy — very lengthy — and on a topic that will only be of interest to a few. The formatting isn't quite how I'd like it either, but then the full suite of BBCodes are not available. Nevertheless, here it is, unabridged.

PART 1

Introduction

I had been delaying placing an order for a .284 Winchester chamber reamer for some months, the reason being that I hadn’t decided on a freebore length to best suit the bullets I intend to use. These will be the 168gr Berger VLD Hunting and Hornady 162gr ELD-match, with perhaps the odd box of 180gr VLD Hunting. The barrel blank for the initial rifle project, brass, dies and neck bushings I already have here, ready to go.

One way to decide the freebore would have been to email the reamer manufacturer with the chambering and bullet choice/s and ask for their advice. If I was U.S.A.-based I could, with little effort, send in a dummy round. This however bypasses developing any understanding of the tradeoffs being made. So I decided I would instead prepare some scale drawings depicting chambers cut with different freebores, and where the profiles of the .284Win brass and the three bullets I planned on shooting – also drawn to scale – could then be superimposed to correspond with either differing degrees of placement into the rifling or differing amounts of jump. The idea was to see, using these drawings, what freebore lengths would maximise contact of the bullets with the case necks, while perhaps also trying to keep the bearing surfaces clear of the neck-shoulder junction that may interfere with neck tension. I did realise from the outset that a given freebore would always be something of a compromise across all three bullets.

This exercise was successful and I arrived at a dimension that will serve my requirements. At that point I really should have called it a day, purchased the reamer, and moved on.

In the process of looking into this question of freebore as part of a reamer specification, I must have trawled through well over one hundred different online forum threads relating to questions over choosing appropriate freebore lengths for a variety of cartridges and bullets. Because of the typical brevity of the posts, both the queries themselves and the answers often included only a subset of the parameters necessary to draw correct conclusions, so weren’t of much use to me, misleading at worst. I was left with the feeling that it would certainly be a useful resource if someone in the shooting community would go to the trouble of presenting a more complete picture of this topic. Since it turned out that some useful seating depth and cartridge length dimensions could be taken from my drawings, I extended the investigation a little and have copied a few of these below with some observations to share with forum members. This post follows the journey to being in a position to make a choice in length of freebore for the.284 Winchester reamer, identifying the parameters needed to make such a decision along the way, and concludes by summarising these with equations. I’ve added some examples that demonstrate the calculations made.


Disclaimer

While having the information I’m presenting here reviewed, it was pointed out to me that calculating appropriate freebore lengths for use with given bullets as part of chamber reamer design is an area that is already well understood within the industry. This raises the question of whether this article attempts to lay out details of things that are better left for those in the industry to resolve on a customer’s behalf? I’m not sure what the answer to that is, but am inclined to the view that knowing more about a subject is generally a good thing. Anyway I would like to make it clear that there will be no revelatory findings in this post. This has been purely an on–paper exercise. I’m not making any claims that particular freebores are going to produce chambers that shoot better. The idea was only to lay out the full set of parameters that need to be included in an equation to determine freebore length and how these may be added or subtracted together to complete this calculation. Where one or more of these parameters must necessarily have values chosen before any calculation of freebore length can be completed, I have either used the standardised SAAMI or CIP values, or if some flexibility is available, opted to use the typical values found on existing reamer prints. In this latter category are dimensions such as freebore clearance (as opposed to length), neck clearance, throat angle, and case mouth angle. The figures allocated to these have been done so without commenting on their suitability for any given application.


Sources for the drawing dimensions

There are a few preparatory things that need to be said before presenting the scale drawings. First up, where have the dimensions used come from? For the cases, these are from the QuickLOAD library and were then checked against a couple of other sources. Fortunately case dimensions are standardised so I didn’t find any variation. On the other hand the bullet profiles did vary between measurements made on bullets I had on-hand and for the VLD bullets the dimensions I cross-referenced against in the sources linked to here:
https://appliedballisticsllc.com/wp-...st-Results.pdf
https://bergerbullets.com/informatio...erence-charts/

Below are these lengths attached to depictions of the 168gr VLD; the Hunting variant for Berger’s figures and mine, and the original target profile from Bryan Litz. Similar variation exists for the 180gr VLD design. A note here that the dimensions added to these and all later drawings have been marked up only in inches without their millimetre equivalents so that they don’t become too cluttered. Though this won’t be apparent from the drawings that will follow, I’ll also mention here that they were all sized in the graphics editor for one pixel per one-thousandth of an inch to simplify keeping everything to scale.



Given the variation I decided to work with the dimensions measured here for all three types of bullet, as these are the bullets that I will initially be using. The differences in the nose length in particular was a reminder as to why it is well advised to use your own Cartridge-Base-to-Ogive (CBTO) gauge measurements in preference to Cartridge-Overall-Length (COL) for the better consistency available in seating depth, and where the ogive should be referenced close to where actual contact with the rifling will occur. The length of the nose of the Hornady 162gr sampled from the boxes here averaged 0.761” and the bearing surface (shank) at 0.460”.


Will the drawings depict loading into the lands or jump or neither?

When loading for the VLD bullets that I use for shooting paper, I am planning on experimenting with seating depths ranging between 0.005” through to 0.025” into the lands, starting with the longest COL and then backing out the bullets. For the ELD-m the reverse direction will be applied, starting with 0.030” of jump and then progressively seating the bullets further out in the case necks so they will be presented closer to the lands as I look for any changes in group size of significance. The chosen freebore must also cater for this variation.

I decided it would be best to set up the drawings to depict the bullets all at a common reference position, and that is for “just touching” the rifling. Adjustments for offsets for jump or to picture loading into the lands can then be made later if required. Since repeated reference will be made to this bullet position I’ll dispense with the inverted commas and try to refer to this as touching or the touch point.


Referencing the bullet to the chamber

With the dimensions of the various elements of both the case and bullets now known and drawn up, it remained to determine the relative position of the bullets to the chamber to achieve a just touching position. Unfortunately the touch point itself is not at a location that either a bullet or chamber reamer manufacturer alone can specify, as its position is a product of the combination of the geometric profiles of both.
The position of these touch points would also need to be referenced back to locations that are dimensionally well defined; one on the bullet under consideration, and one at a point within the chamber. The start of the nose seemed an obvious reference position on the bullets for these points of contact that would presumably be a little further up the ogive, and the front end of the freebore was selected as the reference position in the chamber.

Here is a cross-sectional diagram that shows a bullet just touching the lands, but with the shapes and angles exaggerated to more clearly depict the two reference points referred to above, and the geometry of a chamber throat in general. The bearing surface–to–nose transition of the bullet is marked in orange. The front of the freebore in the chamber is marked in blue. The lands are shown cross-sectioned in the darker blue, and the leade being the tapered portion of the throat forward of the freebore.



The labelled freebore clearance is typically cut to make available a quarter–thousandths of an inch all around the bullet. A common leade seen in many reamer designs makes a 1.5° angle to the bore. Having a limited understanding at this stage of the benefit or otherwise of changing either of these, the plan for now was to stay with these commonly used dimensions when ordering the reamer, so both of these would also need to be incorporated into the drawings. Both freebore clearance and leade angle are configurable when designing a reamer and based on preference. The SAAMI standard .284Win chamber is different in both respects as will be commented on in a moment.

The initial thinking was that the point on the bullet that first touches the rifling – arrowed in red above – could be found, and the distance from there back to the shank–to–nose transition, marked in orange. Then on the chamber side find the touch point to the bullet within the cut rifling forming the leade – the same arrowed location in red – and the distance from there back to the front end of the freebore, marked in blue. With these two dimensions, the relative position of the bullet to the chamber, blue to orange, can be combined with the other case, bullet, and chamber dimensions to calculate both COL and how much neck is holding the bullet, the neck occupancy. A reverse calculation could be used to instead find a freebore length that provides a desired COL or bullet placement within the case neck.

However it turned out that putting a value to this offset was not quite the trivial exercise initially thought. This is the part of a freebore calculation that I found other discussions brushed over; but without understanding how to find this offset and putting a value to it, the drawings – or calculation if just working with the dimensions numerically – cannot be made accurately.


Where is the touch point for each bullet along the nose?

The place on a given bullet that first contacts the rifling, the touch point, will necessarily be where a tangent to the ogive is parallel to the leade. It is worth reiterating this; the bullet touch point will not be where the manufacturer specifies that the bearing surface (shank) ends and the nose begins (the orange location), but some distance forward of this. This touch point can be found by taking measurements on samples of the bullet that will be modelled in the drawings to find where on the ogive it runs parallel to the particular leade angle of the chamber being drawn. I don’t think there is another way of determining this other than by actual measurements on bullets, and that are by no means easy to make with the precision needed. The bullet manufacturers do not provide drawings of their bullets that would allow this point on the nose profile to be found.

Accepting then a degree of uncertainty due to the small dimensions involved and my tools and methods, I measured the 1.5° contact point on both weights of VLD bullet to be 0.006” ahead of the formal shank-to-nose junction (marked up on the earlier 168gr bullet drawings), and three–to–four times this for the ELD-m bullets at about 0.022” ahead, again for a 1.5° leade.


Where is the touch point on the leade?

While measuring tangents on bullet ogives is at least possible, I was initially at a loss as to a way of finding how far up the cut leade from the end of the freebore a given bullet was going to contact the rifling.

So I had a closer look at the geometry to see if I could figure out a work–around, starting with the diagram below where once again the shapes and angles have been greatly exaggerated because the shallow leade angle and freebore clearance are simply too small to depict to scale.

This diagram shows that the touching point must be into the rifling and not where the rifling begins. Perhaps without much thought we accept that the touch point must be forward of the commencement of the rifling when expecting to see the pattern of the lands printed through the sharpie ink or candle soot on a bullet’s nose, if in the past having used that method for determining a reference seating length for touching. If the bullet did in fact contact the throat exactly where the diameter narrows to the nominal bore diameter, indicated by the dotted blue line, the contact rubbed marking would surely instead be a continuous ring around the bullet, or at the least some short lands patterns edged by a ring. An interesting observation, but I digress…

What also may be seen in this diagram is that there is a portion of the chamber between the front end of the freebore marked in solid blue and the commencement of the lands (dotted blue) where the diameter set by the freebore clearance tapers down at the leade angle to the nominal bore diameter, in this case 0.2840”, at which point the rifling begins. The amount of the throat taken up by this part of the taper — between the two blue lines – may be determined with trigonometry from the freebore clearance and the leade angle.



So the position of the commencement of the rifling may be found relative to the freebore, and you will be asking how does this now help find the touch point that initially thinking points to being some indeterminate way further up the leade?

It turns out that I didn’t need to find the position of the rifling touch point directly at all.

If it is assumed that the ogive develops in a circular arc from the shank to at least as far as the touch point, and given that leade angles are always shallow, then a good approximation for how far beyond the start of the rifling the bullet will contact will be half the previously measured distance on the bullet from the start of the nose to the touch point. The other half of this length is marked on the diagram by the orange arrow, and we now have a way of finding the relative positions between the two reference points — the orange and blue solid lines — the above simplification giving a single figure for each bullet–chamber combination to be taken forward into the drawings for placing the bullets.

Just going back for a moment to my 1.5° measurements made on the bullets here, I was initially a little surprised by how close to the start of the nose the bullet touch point was for the Bergers, but then realised I probably wasn’t understanding the VLD’s secant profile. Returning to the information on hand, and having just brushed up on my circle geometry, I used this along with Bryan Litz’s figures for the 168gr VLD target bullets’ secant radius of 18.1 calibres and the meplat diameter he gave to find that the ogive profile should commence with a 3.9° angle to the axis, curving inwards to be 12.4° at the meplat. The figures for the 180gr are similar. Of course it is impossible to swage the jackets to transition abruptly from the 0° shank to this 3.9° angle. The conclusion then is that the VLD bullets must have the 1.5° contact point occurring within the shank-to-nose transition region. Effectively VLD bullets have hybrid ogives then but with a much shorter transition than when a tangential profile is intentionally introduced. These were all insights that I previously hadn’t given much thought to, so already this study – at least for me – was proving worthwhile.



The development of the nose from the shank in the Hornady ELD-match bullet appears intentionally tangential despite the ogive being advertised as secant, and this pushes the touch point further down the nose than for the VLD, as confirmed by the measurements.

A final comment here that finding this point of first contact down the nose for a given leade angle is different from finding the tangent to the ogive of a bullet at a point further down the nose where it has narrowed to a particular diameter – such as at the depth for full engraving, and as has been described in the article linked to here:
https://riflebarrels.com/a-look-at-b...throat-angles/

For the 168gr and 180gr VLD bullets, the tangent to the ogive at the point where the nose diameter reduces to 0.277” turns out to be
4.4° and quite a way away from a match to the 1.5° leade angle that is usually cut for use with these bullets.


A closer look at the .284 Winchester SAAMI design

Turning to the drawings then, and first up here is a cross-section of the SAAMI .284 Winchester chamber with a case in position. Gunsmiths that have just the one .284Win reamer may well have this design. It seemed a little unusual to me when I was preparing the drawing, in that it doesn’t have a conventional parallel-walled freebore. Instead there is a rather shallow leade of 0° 47’ 33” (0.7925°) that continues all the way back to the neck and so creates both the leade and an angled freebore in one continuous taper. The diameter at the rearward end of this freebore is given as 0.290”. A little math shows that a throat cut at the above angle would narrow to the nominal bullet diameter of 0.2840” over a length of 0.217”. I’ve marked that up as the “equivalent” freebore on the enlargement of the region within the dotted rectangle below, and a note here that the reference line for any case or overall length dimension when given will be from the case head / bolt face as is the usual convention.



The case length as pictured is at the 2.170” maximum, meaning the neck is 0.285” long (7.24mm) and this will be used later to calculate the neck occupancy of the bullets. I’m going to disregard the inside length of the neck available to hold the bullet as being potentially slightly longer than the outside. Real-world trim length however is likely to be 2.150-2.160” for safety reasons, shortening the neck. The commencement of the freebore in the chamber as drawn is 2.196” ahead of the bolt face, a dimension built into the grind of the chamber reamer – though this is a figure that appears across many .284Win reamer designs, not just the SAAMI standard.

Not marked on the diagram and independent of freebore considerations but worth mentioning, is the diameter of the neck. The SAAMI chamber is 0.323” at the neck-shoulder junction, tapering slightly to 0.322”. The obvious large amount of neck clearance I’ve pictured in the above enlargement (with a clearly visible gap showing between case neck and chamber wall) is because I plan on running turned necks of 0.014” here for a loaded diameter of 0.312” and the dimensions of the case are drawn to reflect this. Needless to say, the standard SAAMI chamber is not for me, as it would likely work the brass in the necks of my cases too much, but apparently is fine as a no-turn chamber for out-of-the-box Norma brass that is reported as having 0.016” necks, giving a 0.316” loaded diameter.


Testing the modelling by comparison to an existing drawing

As well as the .284 Winchester SAAMI design, Pacific Tool & Gauge also have a drawing on file for a reamer that cuts a chamber with a conventional freebore of a uniform diameter of 0.2845” and with a 1°30’ (1.5°) leade, and significantly with a touch-point that is said to match that of the SAAMI chamber. This freebore length is given on the print as 0.212”. I thought that if I could also show the equivalence of the two freebore lengths with a matching touch point using what had been learned so far, that this would go some way to validating my findings and measurements.

Given their different leade angles, I now knew that the equivalence of the touch points in the two chamber designs would to some extent depend on the bullet nose profile used. I chose to use measurements from the ELD-m for this comparison as being similar to a wider range of bullet profiles near the touch point than the VLD. I then measured on the ELD-m bullets where the ogive would run parallel to the shallower 47” 33’ (0.7925°) leade angle used in the SAAMI chamber and, again within my measuring capability, decided that the touch point would be 0.012” ahead of the start of the nose (instead of the 0.022” I had found when matching to the steeper 1.5° leade reported earlier).

Using the 0.217” equivalent freebore length from the previous diagram, this places the start of the bullets’ nose (my chosen reference position on the bullet) for a touching position in the SAAMI chamber at 0.211” from the commencement of the freebore. That is 0.217” minus half of the above 0.012” figure, following the approximation rule-of-thumb described earlier. This is perhaps a bit difficult to follow without another diagram. As with the earlier drawings, the below also has the lengths and angles exaggerated to make for a clearer depiction – the chamber cross-sectioned in light grey, rifling in the darker blue on the right, and the bullets in a vaguely copper tone. The end of the case neck is shown on the left. Lengths this time are referenced back to the start of the freebore.



For the PTG equivalent chamber, the touching point will be 0.022” ahead of the start of the bullets’ nose, given earlier as appropriate for the 1.5° leade. With the freebore recorded as 0.2845” in diameter and 0.212” in length on the print, trigonometry tells us that a 1.5° tapered leade will narrow the throat down to a 0.2840” diameter at 0.222”, a further 0.010” forward from the start of the freebore. By subtracting 0.011” or half of the above bullet touch point figure we arrive at the same position for the ELD-match’s shank–to–nose transition for touching as in the SAAMI chamber (marked up in red in both). As long as the start of the freebore is the same distance from the cartridge head in both designs (which it is for the two reamers being compared), this confirms that the same seating depth may indeed be used to set ELD-m bullets to be just touching in chambers of both designs.

The PTG print also has written on it that the design should shoot better than the standard SAAMI chamber, something I can’t comment on. Anyway I was happy with this result, indicating as it did that the drawings should give reasonably accurate depictions of how the bullets would align as the throat layout was varied.

PART 2

Drawings depicting each bullet type in the SAAMI chamber

Even though I won’t personally be ordering a SAAMI dimensioned reamer, here are the scale drawings showing how the bullets line up with the drawing for the .284 Winchester case when overlayed to depict a seating position for touching in the SAAMI chamber, and showing the forward portion of the case cut away so the neck occupancy can be seen (and no, I don’t clean the inside of my brass, so the colour is realistic!) What we might be looking for here – if it was the SAAMI chamber reamer we were thinking of using – would be the amount of shank predicted as being held by the neck, the protrusion of the base of the boattail into the case, along with the cartridge overall length for judging fit to the magazine.




In both examples the base of the bearing surfaces are well above the case neck-shoulder junction. While looking at these drawings in the forum posting the amount of contact between bullet and neck can probably only be roughly judged, I’m able here to zoom in and read off the original drawings to the nearest pixel and so can report the neck occupancy as being 0.201” (71%) for the Berger and 0.223 (78%) for the Hornady. Perhaps it could be argued – at least for the VLD – that neck utilisation isn’t ideal with this chamber for a touching position. If either shot well by seating them back for a bit of jump, then this aspect would be improved. The COLs are 3.177” and 3.168” respectively, so if the bullets were to be loaded for some jump, then the revised COLs may be calculated off these figures.

Here is how the 180gr VLD looks for touching. The length of contact of the neck with the bearing surface for this seating depth of 0.274” (96%) looks pretty optimal for the 180gr VLD in the SAAMI chamber, and if the bullet was loaded to push it into the lands, perhaps along with a little throat erosion, the end of the bearing surface should move clear of the shoulder junction where potentially thickening may occur and/or the neck tension may be affected by the reinforcing effect of the surrounding case shoulder.



By the way, my measured lengths for the nose (and boattail) were much the same for the two weights of VLD bullets – within the variation from the finish of the meplat – and so the extra length depicted in these drawings between the two weights of VLD is depicted as entirely from the difference in the length of the shank. That is why the COLs for the two weights of VLD at a given position in the same chamber will always be marked up as the same.


The .284Win SAAMI chambering in short action rifles

As an extension to looking at the position of these bullets in the SAAMI chamber, and because the point is sometimes raised on this forum; if the loaded rounds have to fit a short-action magazine, then the COL will be restricted, and the situation changes significantly. The comment is often made that the .284 Winchester is not suitable for short actions when loaded with the long high-BC projectiles, and these drawings are ideal for visualising the issues that arise.



The above shows where a Hornady 162gr ELD-match will sit in the case when loaded to a short-action magazine length of 2.830”.

At this seating depth:

– the bullet’s nose is extending down into the neck – irrespective of the chamber design,

– there will be a significant loss of usable powder space, as can be visualised from the above,

– the jump to the lands will be over .330” (8.5mm) in the SAAMI chamber.

The situation for short actions improves somewhat if an extended magazine box is used, allowing say for a COL of 2.985” or perhaps a little more. At least the nose can then be seated forward of the case neck. The bullet will still however need to be jumped close to two-tenths of an inch. If thinking about re-chambering a 7mm-08 then consideration should be given to both the action and magazine lengths, and the bullets that will be used. Longer ammunition that doesn’t fit an extended magazine box may still be fed singly through a short action. However another annoyance of a short action is that if loaded any longer than around 3.120” (exact figure depending on the particular short action being used) then the bolt will need to be withdrawn past the bolt stop if rounds need to be extracted unfired, as the bullet tips will hang up in the receiver ejection opening. With a T3 the bolt stop and magazine can both be replaced or modified to match the required length, and so becomes a much better choice if someone can be found who is prepared to use their reamer to re-chamber the Tikka barrel.


Overview of conventional freebore

Turning attention now from the SAAMI to a more “conventional” throat layout when applied to the .284 Winchester, using a uniform diameter of freebore, typically of 0.2845”, and followed by a (again typically) 1° 30’ (1.5°) leade into the rifling, and where there will be the flexibility to vary the length of the freebore. With a perfectly concentric case neck the bullet would then be suspended within a uniform clearance of one-quarter of a thousandth of an inch between the forward portion of the shank and the barrel. The reality must be that for most ammunition, this type of freebore can, to an extent, help straighten a bullet that is not presented true to this front portion of the chamber. The tapered SAAMI chamber cannot achieve this to the same extent when the bullet is jumped and perhaps this may be a contributor to the better accuracy potential of conventional freebore chamber profiles over the SAAMI that has been reported?

The diagram below depicts how it is the length of the freebore that may be used to move the bullet position in the case neck while leaving the position of the bullet ogive relative to the rifling unchanged, setting up the amount of neck in contact with the shank of the bullet, and the protrusion of the base of the boattail into the case (if any), and the cartridge overall length.




A freebore length for heavy-for-calibre bullets

While the 0.233” freebore shown below may seem a somewhat random figure to select to draw, it was referred to by Dave Kiff of Pacific Tool and Gauge in a post on the Accurateshooter.com Shooters’ Forum back in 2013 (post #13) as his choice for use with heavy-for-calibre bullets and “…will also be the conservative start point for the 190 grain bullets”, so I thought I would include this length in the discussion.
https://forum.accurateshooter.com/th...ion-s.3825425/

The COLs given below are my estimates taken from the drawings for bullets that are again seated to be just touching the lands with this 0.233” freebore, and in this position the heavier 180gr VLD (bottom diagram) looks well positioned with the shank contacting 0.249” (87%) of the 0.285” neck, and when loaded into the lands as might be the common practice for VLD bullets, clear of the shoulder junction. For the lighter-weight VLD and ELD-m bullets the neck occupancy figures were extracted off the drawings at 0.176” (62%) and 0.202” (71%) respectively.





I believe the term “conservative” refers to a maximum contact of the bullet shank with the neck at the outset prior to “chasing the lands” as the throat erodes. It is an appropriate place then to note that the positions of the bullets shown and the dimensions given throughout this post are for freshly cut chambers prior to any throat erosion, and the same applies to the leade angles that may well start at the 1.5° or 0.7925° tapers ground into the reamer profile, but will change on usage, moving the touch point for all bullets forward and to an extent outside consideration in this description.

The next set of three drawings is for a freebore of 0.188”, a dimension chosen for the same reason as before. Mr. Kiff had this to say about that particular length: “…for the hand loader that chooses to load 162 and above and wants barrel life then .188 works great. You can burn into the sweet spot and use the heavies.” Below are copies of the drawings predicting where these three bullets would be seated.





The 168gr and 162gr bullets are shown as having 0.221” (78%) and 0.247” (88%) neck occupancy respectively for just touching, whereas for the 180gr the commencement of the boattail is 0.009” behind the shoulder–neck junction, and I predict even jammed won’t move it clear from influence, so some throat erosion will be necessary to move forward of that point as advised.

So what constitutes preferred freebore lengths for Mr. Kiff with respect to these weights and lengths of bullets and that is the basis of these comments? I can only speculate. Almost certainly accuracy potential will be the primary factor and contributing to this would be maximising neck contact for initial bullet concentricity while perhaps avoiding the bottom 10% of the neck where tension variance could occur.

It was now clear from the drawings, and as initially thought, that I wouldn’t be able to optimise both the lighter bullets and the 180gr for neck contact with the same freebore. So I instead chose the freebore length as a compromise between the two lighter bullets and will take the approach I mention here (post #68) for the 180gr by using partially neck sizing:
https://www.nzhuntingandshooting.co....81/index5.html

The final decision was for a 0.180” freebore, a compromise even between the two lighter bullets of similar weight, in part because of the difference between their reported seating preferences. Taken from further drawings (not shown), this length of freebore has the 168gr Hunting VLD 0.081” in front of the shoulder junction when engraved 0.025” into the rifling and the ELD-m is level with the shoulder junction for 0.030” of jump, so not actually clear. These are worse case starting neck placements though, since as mentioned at the outset I will be backing the VLD out of the rifling to find accuracy, and the reverse for the ELD-m where the jump will be reduced. With throat erosion the available clearance from the shoulder will increase. In this respect the freebore choice, particularly for the Hornady bullet, fits the “conservative” description. I also decided to increase the total freebore clearance to 0.0006”. It was pointed out to me recently that there might be good reason to increase clearance if carbon build up will be a concern.

So all-in-all a worthwhile exercise to carry out I thought, and hopefully there has been something of interest so far for a few forum members.

PART 3

Numerical calculations for freebore lengths

PART 1 delves off into the various observations I made along the way, and with the addition of the drawings in PART 2 becomes a bit of a meandering narrative. At the start I proposed bring it all together to summarise what parameters including freebore were related. Here then are those parameters listed, and as long as any combination but one is known, then this missing value may be found from the others. It probably goes without saying to be consistent with the units of measurement, either using inches and thousandths of an inch, or having them all recorded in millimetres. The allocation of the letters to the variables is mainly arbitrary.

A = Length from the case head / bolt face to the start of the freebore,

FB = Length of the freebore,

C = Freebore clearance (one sided),

D = Leade angle,

E = Distance of the bullet touch point ahead of the start of the nose for the above leade angle,

F = By how much the bullet will either be loaded into the lands or jumped,

N = Length of the bullets nose,

COL = Cartridge overall length,

and defining a couple of values derived from the above, let

J = C / Tan(D), being the distance from the front end of the freebore to the start of the rifling, and

K = E / 2

Here is a description of each variable:

A
Length from the case head / bolt face to the start of the freebore

A property of the reamer/chamber design. This figure is usually pretty constant across most designs for a given cartridge. While it might be thought that a varying neck clearance between reamer designs would change this value, in reality variation in the chamber neck diameter (and the case mouth angle if it deviates from 45°) are instead almost always absorbed back into differences in the length of the neck portion of the chamber, while this start of the freebore dimension typically remains fixed. If designing your own chamber for having a reamer made, yes, of course this dimension may be changed, but I’m not aware of any particular reason to do so other than perhaps providing additional space for forming or growing longer necks. In the past I’ve just opted to look out a few prints for the cartridge of interest, ideally including the SAAMI or CIP standard design, and used the figure common to those. For the .284 Winchester that length is 2.196”.



FB
Length of the freebore

A dimension also ground into the reamer. This is commonly the unknown that needs to be found when values for all the other parameters have been set at what are thought to be optimal values, but it may not always be this way. A rifle might for example have a chamber of known dimensions, including the length of freebore, and the purpose of the calculation may instead be to see how close to the rifling a bullet may be seated and have the resulting round still fit in the magazine. I’ve come across drawings for the .284Win with freebore lengths ranging between 0.120” and 0.250” based on application.


C & D
Freebore clearance and Leade angle

Again both properties are ground into the reamer and so transferred through to any chambers cut with that reamer. Use these with trigonometry to find how far forward from the end of the freebore the rifling starts as given below, giving value J. Whether the intention is to stick with industry–typical values for C and D found in the majority of reamer designs, or make changes to either or both of these, both parameters do need to have been chosen prior to carrying through to the calculations that will follow.
By far and away the most common freebore diameter for reamers across many cartridge designs provide a clearance of 0.0005”, or 0.00025” on each side of the bullet, C being the one-sided clearance. Leade angles also vary between cartridges. 1.5° appears to be widely regarded as a good choice for accuracy with high-B.C. projectiles and is used in many .284 Winchester reamer designs. 3° is also common enough amongst other .284 calibre designs, the SAAMI 7mm Remington Magnum and 7mm-08 Remington being examples.



E
Bullet touch point ahead of the start of the nose for the above leade angle

If not using the specific Berger VLD or Hornady ELD-match bullets detailed here (in which case you may choose to use my figures) then this parameter will have to be measured on samples of the different bullets to be used, and this is likely to be the trickiest aspect of the whole exercise. Hopefully apparent from earlier, we are wanting to know where on the nose of the bullet the profile curves inward to match the leade angle D; specifically length E is the distance from there back to the start of the nose. This assessment may be carried out in a lathe by setting the compound to match the leade angle, then marking and measuring off the ogive to give a value to E, perhaps by viewing under a microscope. There are other more “agricultural” ways to take this measurement that can be left up to the ingenuity of the individual to figure out. Halve this value to find K, as discussed earlier being a good approximation for the distance between the start of the rifling and the base of the nose for a bullet that is positioned relative to the chamber for just touching that rifling. Reusing the diagram from earlier below that now has these two parameters marked up.



F
Bullet jump or loading into the lands

In the formula given a little further below, the values for bullets seated to be pushed into the rifling beyond the touch point – lengthening the COL – should be positive, whereas F should be negative for any amount of jump.


N
Length of the bullet nose

If the manufacturer has thoughtfully provided an online table of their bullet dimensions, then the length of the nose, the length of the bearing surface, and the length of the boattail (if present) may all be taken from there. Alternatively these may be measured on samples of the particular batch of bullets to be used to capture any batch variation.
Indicative figures for the bullet dimensions may be found using a combination of micrometer and callipers as shown below. Using this method the micrometer may be set to 0.01mm under the measured bearing surface diameter – any less and it is too easy to engrave the bullet into the micrometer measuring faces with the force of closing the callipers. Keep everything square. The contact point of the bullet with the micrometer should also be on the centreline out from the face centres. For calibres that don’t match the face diameter typically the bullets will need to be shimmed up into alignment. Something similar applies to the callipers – don’t let the depth stem slide around the curve of the edge of the micrometer face when setting for a reading. The available shank–plus–nose and shank–plus–boattail (pictured) lengths are then combined with the overall length to break out the individual bullet dimensions. The above offers an alternative to using a lathe and dial indicator. This and other measurements will benefit from averaging over a reasonable sample of bullets.



COL
Cartridge overall length. No explanation needed.



.284 Winchester worked example

The cut-away diagram below for a bullet seated to be just touching the rifling brings the measurements together and with the summation connecting these also given. Trying to include F on this diagram for both placement into the lands or jump would have made it too cluttered, so it was left out pictorially but is there in the equation. Even so it will be necessary to zoom in on the diagram to clearly see the dimensions depicted by J and K.



As a worked example I’ll put some figures to one of the .284 Winchester loaded rounds shown earlier in PART 2 and see if the same result for the Cartridge Overall Length can now be found only numerically and without the need for the scale drawings and counting pixels. The example chosen will use the Berger 168gr VLD in the chamber with the 0.188” freebore and with 0.0005” of total freebore clearance, and a 1.5° leade. If scrolling back to find this, it was part of the last pictured set of loaded rounds. To demonstrate changing the bullet seating, the VLD can also be pushed forward 0.020” into the lands. The predicted COL determined from and marked up on the drawing was 3.157” with the bullet loaded for just touching, so we should end up at 3.177” when the bullet is shown pushed twenty-thou into the rifling.

The starting point is a value for A, given earlier for the .284Win as 2.196”.
The freebore FB is 0.188”.
Using a pocket calculator to enter C as 0.00025” and D as 1.5° results in J = 0.010”
Given earlier, E for both VLD bullets with a 1.5° leade was measured as 0.006”, so half of this gives K = 0.003” (making it difficult to see above without enlarging the image)
For nose length N I’ll continue to use measurements made on bullets here and reported earlier as 0.766”

Plugging these values into the above formula:

COL = 2.196” + 0.188” + 0.010” – 0.003” + 0.020” (positive for into the lands) + 0.766”

= 3.177”


While being able to calculate the COL is of interest, freebore is the chamber dimension used to set the bullet position in the case neck while leaving the position of the bullet ogive relative to the rifling unchanged. So it is appropriate to look at the summations that need to be made to predict the amount of neck that will be in contact with the shank of the bullet, and the protrusion of the base of the boattail into the case (if any) for a given freebore — doing this now from the numbers rather than resorting to diagrams.

Start by getting some values for where the neck starts and finishes. The most convenient reference point for these is back to the case head as this reference is common to the reamer as the bolt face (setting aside chambering headspace variation for now). The Overall Case Length and the Case Head to the Neck–Shoulder Junction can then be taken from dimensioned SAAMI or CIP case drawing of the cartridge of interest. For the .284 Winchester these lengths are 2.170” and 1.885” as shown below, and labelled as M and P respectively. M will be given only as the maximum case length. Real world neck trim lengths will shorten this a little. So M minus P gives the maximum available length of the neck. Given the variation in trim length there is little issue with disregarding that the interior of the neck will be very slightly longer than the outside measurements indicate. For the .284Win the difference between the above two lengths NL is 0.285”



Assuming the freebore length is known and a calculation has already been made to find the COL for the preferred bullet position, then deducting the length of the bullets’ nose, N, and shank, S, from the COL will be the simplest way to locate the base of the bearing surface relative to the case head, and so leading to a figure for neck occupancy.



Now carrying on with the above example using the Berger 168gr VLD and plugging values into the formula, COL – N – S

3.177” – 0.766” – 0.442” = 1.969”

The figure of 0.442” used here for the length of the bearing surface of the 168gr VLD, like the length of the nose, was taken from measurements made on samples of the bullets here and was the figure incorporated into the drawings. Berger’s published value for the Hunting variant of VLD is 0.417”. Berger advise that all published dimensions are “…for reference only. They will vary from lot to lot, but will be consistent within a lot”. Measuring actual samples of the bullets to be used from out of the box seems a good idea to capture any variation.

With the base of the bearing surface located, its position relative to each end of the case neck may now be examined. When comparing the shank-to-boattail transition at 1.969” to the end of the neck at 2.170” the difference between these shows that the bullets will be held by:

2.170” – 1.969” = 0.201” of neck.

Again this is an exact match to the earlier drawing, given that in this example the bullet has also been pushed forward 0.020” into the rifling rather than the touching placement that was depicted in the drawing. The clearance to the neck–shoulder junction (where the dreaded donut may lurk) will be the difference in the other direction:

1.969” – 1.885” = 0.084”.

What constitutes sufficient clearance from the area of potential internal neck thickening will presumably vary based on the extent of that same thickening if present. An educated guess would say that the 0.084” calculated here should be more than sufficient.

Since the drawings have lead to a sufficient understanding of how everything fits together, and the same dimensions may be found both pictorially or numerically, future assessments could dispense with the diagrams..


Transferring the calculations to other cartridges and bullets

These calculations can be transferred easily enough to other cartridge designs and used to calculate preferred freebore lengths with respect to neck occupancy and predict COLs with different bullets in the same way. That may be where much of the value of the above lies rather than specifically for the .284 Winchester.

So I’ll finish with a completely different set of calculations based on a 7mm Remington Magnum example that will also require shuffling the formulae a little. This time the task returns to the question I initially had of finding the length of a conventional freebore to specify when ordering a reamer. I’m going to be target shooting with my 7mm using Berger 180gr hybrid target bullets. The leade will be a non-standard value for this cartridge of 1.5°, perhaps better suiting the hybrids than the regular 3°, and the freebore diameter will be 0.2848” allowing extra clearance to cater for lazy cleaning practices when using dirty double-based powders.

The hypothetical requirements for this example will be:

1. set the freebore to provide a 90% neck occupancy when seating the hybrids for 0.020 of jump, and

2. the finished rounds must have a COL of no greater than 3.580” even if the bullets are then seated out to touch the rifling – to keep within the available 3.60” magazine length of the donor action.

Note that it may not be possible to satisfy both of these together. We can’t tell until the calculations are some way to being complete. If restrictions like this are placed on the COL then long bullets will necessarily have to be seated back into the case to meet these, possibly requiring that the bullets occupy the entire neck and beyond with the bases pushing back into the body of the case. No tweaking of the freebore length can correct for this.

Let’s start by collecting the data needed.

I found case dimensions for the 7mmRM in several places. The case image shown below on the left came from here:
https://sierrabullets.wordpress.com/...num-load-data/
The dimensions that aren’t relevant have been removed from the images. This particular reamer print on the right was found here:
https://precisionrifleblog.com/2012/...chamber-print/
From these two prints we get the parameters listed down the LHS below.



Having measured a few of the bullets from a part box here they seem to differ very little in shape from the values given by Berger, so the Berger values will be used from the table linked to earlier. The 180gr target hybrid details have been cut and pasted from that page below.



The Freebore clearance C, and Leade angle D, may now be used to calculate how far forward from the end of the freebore the rifling starts J.

J = 0.0004 / Tan (1.5°)

= 0.015”

Now we come to the tricky bit; measuring where on the bullet the ogive runs parallel to the 1.5° leade. I’ve measured this on some hybrids here and for the purposes of this example we’ll let E be 0.058” up from the commencement of the nose, so

K = E / 2

= 0.029”

That’s it. Time to crunch some numbers.

90% occupancy of the neck of length NL = 0.271 places both the base of the bearing surface 0.027” (10%) in front of the neck–shoulder junction at P and 0.244” (90%) back from the maximum length of M. Either may be used to place the start of the boattail at 2.256” from the case head.

2.256” = 2.229” + 0.027” = 2.500” – 0.244”

We may as well at this point go ahead and add on the length of the bullets’ bearing surface and nose to give a value to the COL to see if criterion 2. is to be met. This will then be:

2.256” + 0.430” (shank length) + 0.852” (nose length) = 3.538” from the case head.

Given that the 90% of neck occupancy was for bullets that were seated back for twenty-thou of jump, and 2. allowed for some seating flexibility up to a just touching position, we can see that the maximum COL the magazine must cater for will be 3.558”, which lies just under the specified 3.580” limit. So requirement 2 will be met.

The case neck occupancy requirement sets the COL without at this stage having needed to consider the chamber dimensions that include the freebore. Let’s carry on and calculate the required freebore length for the reamer given that the 90% neck occupancy position must also be providing 20 thousandths of an inch of bullet jump to the rifling.

Rearranging the original formula of COL = A + FB + J – K + F + N to make freebore FB the subject of the equation, and where F will be a negative value to represent bullet jump.

FB = COL – A – J + K – F – N

= 3.538” – 2.540” – 0.015” + 0.029” – (–0.020”) – 0.852”

= 0.180”

Transferring these dimensions to drawings of the chamber, case, and bullet, scaled as before to one pixel for each thousandth of an inch, allowed me to see if everything lined up as expected. Given that the diagram is just a pictorial method of making the same summation, the result is the same. Below is that drawing. I’ve tried to mark up as many of the contributing lengths as I can, but it is not possible on this diagram to realistically depict the smaller values in the region of the throat; that is J, K, and the jump F, so you’ll just have to take my word that their relative positions were all as they should be.

PART 4

Verifying the freebore length

Having decided on the chamber dimensions, ordered and received the .284 Winchester reamer and had a chamber cut, it is of course of interest to then try and confirm if the grind matches the print. Measurements may be taken off the reamer. I decided instead to try and measure the cut chamber, at least to the extent of checking the location of the leade. The thinking was that if the leade was found to be as expected, then the freebore length will likely also be correct.



For this I purchased some custom carbide pin gauges, requesting that no chamfer be applied to the ends. These are ground to a ±0.001mm tolerance and the diameters of interest were 7.214mm (0.2840” ) and 6.706mm (0.2640” – for checking 6.5mm chambers) These may be loaded into the case necks as a bullet would be, and the overall length set to position the front of the gauge so it just touches the leade where it narrows to the nominal bore diameter — the point indicated by the blue dotted line in some of the earlier diagrams. I drilled and tapped the flash hole of a .284Win case and used a machine screw to set the protrusion. This also allows for the case and gauge together to be pushed back out of the chamber with a cleaning rod if needed without risking altering the position of the gauge in the neck.



This measurement doesn’t allow for the headspacing to be separated out though, so if the chamber has been cut as it should, and lies in length between Go and No Go gauges, then an extra uncertainty of up to say ±0.004” will be overlayed.

The pin gauges, being harder than the barrel steel, can damage the throat if not used with care. The case loaded with the gauge was first checked and corrected for concentricity, and then the progress of presenting the gauge to the throat was followed with a forward-viewing borescope. I used a Teslong NTG100 for this. Below left is an image of the face of the gauge sitting partway up the freebore with the 0.0003” freebore clearance showing between them. In the middle image the pin gauge has been advanced to the just touching position and the bolt closed on the case. The leade formed by the lands cut at 1.5° can be seen ahead of the gauge. The rifling is a 5R profile.



On the right is an image taken after the case and gauge were withdrawn and where I added the right-angle mirror to the borescope. This shows very light contact marks (arrowed) where the gauge has scored the base of the rifling, confirming contact with the throat. The contact marks did not extend across the grooves confirming that these have been cut to no less than the 0.2840” bore diameter. The length of the extracted case and pin gauge when measured with (metric) Vernier callipers was 60.65 ± 0.05mm, or 2.388”.

For a .284 Winchester chamber cut with this reamer, having a 0.180” freebore, 0.0006” total freebore clearance, and a 1.5° leade, the overall length of the case and gauge combination should, using the variables previously identified, be:

A + FB + J

2.196” + 0.180” + 0.011” = 2.387” (the 0.011” figure for J is from the 0.0006” total freebore clearance)

I was surprised that this measurement produced such a close match for the calculated length, though perhaps I shouldn’t have been, as JGS have a reputation for precision in the grinding of their reamers. It looks like the barrel has been fitted with the headspace mid-specification too. It goes without saying that meaningful measurements of this type can only be made on an unfired chamber before any throat erosion has occurred.


I think we have space for one more worked example. I have a reamer here ground for 6.5x47 Lapua that I purchased in 2018, and prior to developing an understanding of how to crunch the numbers to match freebore length to neck occupancy for a given bullet. I just went with the general consensus for a freebore length found online, and that, only by coincidence, was the same as for the earlier .284Win, at 0.180”.
I also have on-hand a couple of unfired chambers cut with this reamer that could be checked with the 0.2640” diameter pin gauge. Let’s now see if the choice made in 2018 was a good one?

First up the necessary information is gathered. Bullet dimensions come from Berger again, with my preferred bullets being the 130gr and 140gr VLD. I’m going to take the easier option and just use the published lengths rather than measure bullets from out of the boxes that are on-hand.



Neck dimensions come from any one of a number of different sources.



Here is the reamer print with just the dimensions of current interest retained.



Because it has been found that for VLD bullets the touch point to a shallow leade lies so close to the front end of the bearing surface, there is also a further short cut that may be used. The error for using the same 0.006” figure as for the 7mm VLDs, if present, will be small. So I’ll go ahead with using that value, which will still be perfectly acceptable while bypassing further tedious measurements on the bullets’ noses.

Applying the same letters to the above variables as in Part 3 earlier, and looking at the 140gr weight:

A = 1.8744”, length from the case head/bolt face to the start of the freebore, found across a number of reamer prints online,

FB = 0.180”, length of the freebore,

C = 0.0003”, freebore clearance (one sided),

D = 1.5°, leade angle,

J = 0.011” = C / Tan(D), being the distance from the front end of the freebore to the start of the rifling,

E = 0.006”, my approximation for how far forward of the start of the nose these bullets will contact the leade,

K = 0.003”, = E / 2, the approximation for the distance between the start of the nose and the beginning of the rifling for a bullet that is just making contact with the leade,

F = zero, bullet jump or seating for into the lands. Start by assessing COL and neck occupancy for a bullet that is just touching, then consider these offsets once the initial numbers are known,

N = 0.731”, the length of the nose of the 140gr VLD bullet,

S = 0.435”, the length of the bearing surface of the 140gr bullet,

M = 1.850”, case maximum length,

P = 1.548”, case head to the neck–shoulder junction,

NL = 0.302”, = M – P, the maximum length of the neck,

COL = the current unknown.

COL = A + FB + J – K + F + N

= 1.874” + 0.180” + 0.011” – 0.003” + 0.731”

= 2.793”


Now finding the distance from the case head to the base of the bullets’ bearing surface when seated to be just touching. I realise I haven’t allocated a letter to this value, but that is okay.

COL – N – S

= 2.793” – 0.731” – 0.435”

= 1.627”



The amount of neck holding the shank can now be determined by comparing the base of the shank at 1.627” with the front and rear of the neck at 1.850” and 1.548” respectively, all lengths referenced back to the case head. So it is found that the 140gr VLD will be occupying the front 0.223” (74%) of the available 0.302” of neck, while the unoccupied portion of the neck is 0.079” long. Thinking now about whether I will be jumping the bullet and I could seat the 140gr further back into the case by 40 thousandths of an inch and still be clear of the neck shoulder junction, while in the other direction for a loading into the lands, say 20 thou, and the shank is still gripped by two-thirds of the available neck.

It certainly looks like the 0.180” freebore was a sensible choice then for a 140gr VLD. The assessment for the 130gr VLD follows the same steps with the expected loss of neck occupancy from the shorter shank being confirmed, but no need to document the calculations.


The method of checking the position of the front of the freebore with a pin gauge was also applied to one of the two chambers using the 6.706mm diameter gauge. The length from the “loaded” round, case head to the end of the gauge, was 52.70 ± 0.05mm, or 2.075”.

The expected position for the start of the rifling will be A + FB + J as before.

1.874” + 0.180” + 0.011” = 2.065”

So a 10 thou difference, where contributions to this offset would come from some combination of the reamer grind, the head spacing, and perhaps that slight burrs were raised at the base of the lands with the gauge, indicating that it may have been seated in the case a little on the long side. These were slight and polished out with JB and little effort.

Here are some borescope images of the 6.5x47 chamber. On the left is the seated pin gauge snugged up to the start of the rifling, bolt closed. The middle image was taken after the gauge had then been withdrawn and shows the burrs. The RHS image was taken with the borescope facing in the opposite direction up the barrel. In the middle and right images the transition from the barrel makers longitudinal tooling marks in the cut rifling grooves over to the axial marks from the reamer can be clearly seen — even if the burrs weren’t there to highlight this.

PART 5

Verifying the Cartridge Overall Length predictions

Measurements for some cartridge overall lengths can now be taken for actual bullets seated to be just touching . These can then be compared to the COL predictions. Given that the pin gauge has been used to zero out the position of the leade, if then say a longer than expected COL is measured, the cause should be limited to either the bullet nose being longer than the earlier measurements had found, or the touch point on the bullet being further back and closer to the shank-to-nose transition. Remember that finding these touch points had relied on measuring tangents to very small angles (under a microscope here) and the overall calculation relied on at least one geometrical approximation. That said, with the VLD bullets, an error in the touch point, already only 0.006” ahead of the shank, will be unable to make a significant contribution.

The reality is that taking accurate just touching COL or CBTO measurements can be – like measuring the tangents to the ogives – another tricky endeavour. When the rifle action allows, I prefer – along with many on here – to use the “Bolt Close” method popularised by Alex Wheeler. The available .284Win chamber is on a Mauser-style action. With the extractor claw and collar removed it was still not ideal for using with this method. Instead of relying solely on the fall of the bolt to detect contact between the bullet and lands, the amount of stiction when pushing the bullets back out with a cleaning rod was also used. For the 168gr Berger VLD in the .284Win chamber I was expecting the following COL:

COL = A + FB + J – K + N

= 2.196” + 0.180” + 0.011” – 0.003” + 0.766”

= 3.150”

Instead my best estimation found 3.156” for just touching.

The initial thinking was that my tinkering with the pin gauge might have removed a bit more of the lands in the start of the leade than I had thought, and affected this measurement. On reflection and from reviewing the earlier diagrams it becomes clear that the initial touch point with the bullet should typically be forward of any scuffing at the commencement of the rifling. While measurements with the VLD bullets may perhaps be affected, the ELD-m should be touching further up the rifling. Plus the gauge contact had been very light. Let’s attribute this to probable nose length variation — slightly longer noses on these bullets used over those I had taken the original measurements on (3 years prior).

I then seated some 162gr ELD–m. The differing nose length and touch point to the VLD meant a revised COL of 3.137” was expected.

COL = 2.196” + 0.180” + 0.011” – 0.011” + 0.761”

= 3.137”

This was the same figure that was measured.



One of the 6.5x47 chambers is on a Barnard P that has always been found to be pretty good for applying the Bolt Close method for detecting bullet contact with the rifling once the bolt was stripped down, so the same method for validating the COL could be applied. A number of 140gr VLD projectiles were seated in cases and I was satisfied that the average of the just touching COL measurements was 2.787”

The predicted COL using figures off the reamer print was calculated a little earlier as 2.793”, the actual measurement then being six thousandths of an inch shorter than this. However my subsequent pin gauge checking had pointed to the leade being forward of the expected position by ten thou, so in reality the COL was 0.016” less than expected.

With only a couple of possible reasons for this error, I went ahead with checking some bullet nose lengths. Recall that the COL calculation used the published Berger value for the length of the nose. By seating several bullets a few thou longer than touching and chambering several times I was able to work up some consistent scuff marks on the jackets as pictured. We can safely treat these as being symmetrical in length about the touch point, which itself is being treated as 0.006” ahead of the base of the nose.



I measured 0.710” from the tip of the nose to the centre of the scuff marks, leading to the conclusion that 0.716” was the better figure to use for the length of the nose for these particular 140gr bullets rather than the published figure of 0.731”, and so with these bullets being 0.015” shorter than expected, the above offset was explained.


Well that’s it from me on the topic of freebore. If you have read this far that is an achievement in itself. As I wrote at the outset, I couldn’t find anything like the sort of information presented here elsewhere online. Probably it is well document historically in books. The NZHS Forum offers the flexibility to post this sort of thing and it doesn’t matter if it is only of interest to a few — as there aren’t any circulation or sales figures to consider.