Seating depth

I am using a Hornady OAL gauge, with the 223 modified case, and the barrel that clips on to the calipers and measures the ogive for my seating depth.
My question is will it make any difference to the measurement if the cases are slightly different sizes or do they have to be trimmed all to the exact size so the cases are seated at the same measurement?
Because I got the seating depth at a set size, then measured from the base of the case to the ogive after i seated every round and they are coming out different measurements, only slightly.
For example, I wanted to seat my bullets at 1828 and some would be anything between 1822-1836.
I am guessing this is the cases being slightly different sizes.

This is probably easy to understand, but I am having trouble so apologies if this is a stupid question:pacman:

Er, the Hornady AOL gauge (comes in straight for bolt or bent for semi variants) is used together with the modified case to determine where the lands are for a particular projectile...

The "barrel" thing you talk about uses either a bullet comparator to give you a CBTO measurement, or a headspace comparator to measure case base to shoulder datum.

Case length (unless it is REALLY wrong) should not affect ogive measurements.

14 thou difference in CBTO is not slight, that is a big variation.

Take the hornady bits off your caliper, measure the case lengths...

Put the bullet comparator on, and measure your projectiles, from the base of the projectile to the ogive...

One of those should tell you where the variation is.

Take 3 projectiles, use the AOL gauge and modified case and get an average of where your lands are, subtract 0.010 from that and use it as the starting seating depth.

The seating stem shape and projectile nose shape can sometimes combine to stick projectiles in the stem and pull them back out a bit, you should be able to feel it stick as you back off the press if that is happening tho.

Technical. I just push one up the spout out long and know that that's at lands. Then work back from there.

For my swede that gives me about 1mm of grip between the case and the bullet (140gr SST) :(

Quote:

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Originally Posted by ebf

Er, the Hornady AOL gauge (comes in straight for bolt or bent for semi variants) is used together with the modified case to determine where the lands are for a particular projectile...

The "barrel" thing you talk about uses either a bullet comparator to give you a CBTO measurement, or a headspace comparator to measure case base to shoulder datum.

Case length (unless it is REALLY wrong) should not affect ogive measurements.

14 thou difference in CBTO is not slight, that is a big variation.

Take the hornady bits off your caliper, measure the case lengths...

Put the bullet comparator on, and measure your projectiles, from the base of the projectile to the ogive...

One of those should tell you where the variation is.

Take 3 projectiles, use the AOL gauge and modified case and get an average of where your lands are, subtract 0.010 from that and use it as the starting seating depth.

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Sorry for the typo on the AOL gauge ebf, I see you picked that up. Slightly anal, you knew what I meant.
I do everything you have suggested anyway. My question mainly was if the cases are all different sizes will that make any difference to the ogive measurement when I use the comparator on the caliper to measure from the base of the case to the ogive?
Because my cases are all different sizes. My trimmer only trims to one size, I don't have a trimmer that is adjustable yet, it is on it's way though.

It's OAL not AOL, I have dyslexia, what can I say :D

So where is the variation, in you case lengths, you projectile measurements, or somewhere else ?

It is probably more to do with variations in the projectile shape. The length of the case won't make a difference.