Thread: When to re optimize seating depth ?

This topic is pure puffin bait @Bagheera. I sat on my wings for a week but...

Referring to both your related threads, to me it would make sense to re-look at this for anything that significantly changes the two factors between which cancellation is trying to be achieved: muzzle movement and barrel time. Adding or removing a suppressor obviously, bedding changes, barrel length, fluting, etc, and then for barrel time – well anything that results in a measurable change in muzzle velocity, or is suspected as having changed barrel time without necessarily changing muzzle velocity, such as a change in powder burn profile.

This said the purpose of ladder testing seems to be widely misunderstood. I’m going to digress here, and if this gives cause for thought or discussion here rather than wasting time and ammunition at the range then well and good. The original writing - and also those of Newberry on his OCW where the end goal is the same - are quite clear in describing what the purpose of the testing is. I’m not sure why anyone would run an experiment if they weren’t clear on what was being investigated? The original purpose of ladder testing was to find a range of powder charges over which the movement of the GROUP CENTRES on the target is at a minimum. Group centres are estimated with single shots. In OCW testing group centres are estimated by shooting groups. There may be several ranges over which minima occur. And a reason you would want to find this range/s? Once a ladder node has been found then the thinking is that variation in the ammunition will have a lower impact on the change in position of the shots on average than it might otherwise have if ammunition was loaded to be away from a ladder node. Typically a ladder node was used to allow the care in measuring powder charge to be relaxed when reloading.

Circular group size and ladder nodes are INDEPENDENT of each other. It is possible to have a minimum in the movement of group centres over a range of powder charges - a ladder node - where the group size is appalling. A grain away the group size could tighten right up, but not be at a ladder minimum. The choice here would then be either to operate at the ladder node and spray the bullets all over the target but know that the group centre is robust, or place all the shots closer to the intended place on the target, but with a requirement to be more consistent with your reloading practices to achieve this. Not really a difficult decision I would have thought.

Originally Posted by Bagheera

The theory is that you try to adjust bullet velocity so it exits the muzzle while it is more or less stationary.

I see this as part of the picture since it applies in the horizontal plane. Vertically there also needs to be cancellation with the rise in POI from the shortening flight time as powder charge is increased. So in the vertical axis the barrel needs to be launching the bullets at progressively lower angles as powder charge increases. If separated into horizontal and vertical nulls, then these may occur over the same range of powder charges - or they may not, an issue in itself.

To make a contribution to the original question I’m thinking how seating depth, throat wear, bullet batch, and powder batch fit into this picture? From experience the radial spread of shots can be quite sensitive to these factors. They would affect a ladder null only in as much as they impact the above mentioned cancellation. Bullet type would if it changes both barrel time and muzzle movement on firing, so different bullets most probably will.

A problem with ladder testing stems from there being only one shot taken for each powder weight. From a statistical point of view this is problematic since the shooter is trying to determine the movement of group centres as powder charge is changed, but attempting to do so with only one shot per charge weight. That is why the OCW method is better. With a conventional Audette ladder the variation in the position on the target for individual shots resulting from the normal spread we characterise under circular or radial group size will IN MOST CASES swamp the movement that the ladder test is being used to find. A bad situation can possibly be improved by shooting at longer ranges, but unless the rifle is known to shoot very tight groups right across the load range of interest then nulls when they are found are likely to be spurious - the result of chance bringing 2-3 shots into close proximity on the target.

Here are two interesting questions to ask: are the groups from the rifle whose loads are trying to be improved by ladder testing, circular in distribution or elongated (in any direction). If circular then ladder testing will not be of any help. Why? Because the theory says that when a load is away from a ladder node that shots are going to have a tendency be strung out on the target along a line caused by variations in the position of the muzzle at the various times the bullets exit as a result of variations in the ammunition. I use “tendency” since the other NEARLY ALWAYS MORE SIGNIFICANT causes for the spread of shots on a target will be superimposed over this pattern. This movement is not random. The basis of ladder testing and finding a ladder node is that this component to the spread of shots on a target is predictable, that shots made with IDENTICAL ammunition will no longer have this contribution to spread. The issue trying to be addressed is that ammunition CANNOT be made identical and to try and minimise the affect of variation in the ammo on the target. If plotted, the muzzle positions at the time the bullets exit will form a continuous line. So variations in ammo will result in the pattern of shots on the target mirroring this movement. Groups will have a smeared elongation component. If they don’t, then there is no point looking for ladder nodes because this aspect is not making a significant contribution to spread.

Secondly: does the rifle shoot its ammunition with tight SD around the average velocity? It is a reasonably safe assumption that this will also mean a tight distribution in barrel times, and therefore consistency in the time that the bullet exits the barrel, and so consistency in the exact direction the barrel launches the bullet in. If this applies, then I don’t see any reason to attempt a ladder test. Ladder nodes are places to operate to minimise the effect of variations in ammo. If you don’t have much variation in your ammo then the usefulness in ladder testing is doubtful.


Personally I don’t have a single rifle in the safe where I can demonstrate that ladder or OCW nodes have been any help to me in shooting smaller groups. The reason ladder testing enjoys such a high profile appears to me to be because it doesn’t use much ammunition to run, seems to be successful in providing results in the form of clusters of shots (when actually these “nodes” are almost always specious) and - as yet another reason for all our shots not going into the same hole - has an understandable theory behind it when much in the science of accuracy remains unexplained.

It is often said: don’t put forward problems without coming up with solutions. Shoot for group size, 5-6 shots each time, looking for circular distribution minima. Load as consistently as possible and quantify the results with a good chronograph.

Well that was fun. Time now though to jump off one of my favourite hobbyhorses - hope I haven't upset too many people along the way...