OAL/seating depth/final case volume?

[Nuke8401]

Please forgive me if this has been answered before. I reload 9mm and several others. Using OAL seems very inaccurate when seating depth or final case volume would seem to be the true measurement needed. For instance I would guess that 124gr JHP from different manufacturers when seated to get the same OAL would result in different seating depth and resulting case volumes. How much of an impact could/would this have on the resulting pressures?

Am I sweating the small stuff here?

 

[rcmodel]

Yes you are.

OAL, as listed in the reloading manuals, gives you the exact seating depth used for the pressure testing.

It also gives you a fair chance of having the specific bullet profile feed in most guns.

That is why it is best to use load data from the specific bullet manufacture if they publish it.

The other thing is, even a reloading caveman has the tools & skill to measure OAL accurately.

Not many of us can measure remaining case volume under a seated bullet, or the actual amount of bullet shank seated inside the case.

Yes, there are ways to do it, involving water, and CC measuring devices, and pulling marked bullets, etc. But nobody in their right mind is going to do all that if they can get the same results in a few seconds with a dial caliper.

rc

 

[Walkalong]

Probably, but I do record the length as well as the diameter of all the bullets I load. That way I can figure how deep in the case it is if I want to. It is one small thing to keep in mind when loading.

It is almost a non issue with traget loads in .45 and .38 Spl with the large case and low pressure, but it can come into play with a small case and higher presurres like the 9MM.

So yes, it makes a difference, but it isn't worth pulling your hair out over.

 

[The Bushmaster]

I've never really worried about that much at all. I have done tests to see what difference seating depth makes on a given bullet and found that you really have to have large variances to make a difference. Granted, my test used two pieces of test equipment (Calipers and a Chronograph). If you are loading the same bullet profile as the manuals chosen bullet, it's doubtful that you will have a problem.

A rough idea of what the remaining volume would be to seat a bullet and take another (unseated) bullet and compare it with the seated bullet.

Looks like you have received three good answers already...

 

[Nuke8401]

Thanks for the replies,

I guess with the target loads I make there is low risk. I have loaded some 7.62x39 for my Ruger mini-30 and had to weight the charges several times as I was surprised/concerned how full the cases were with the full charge. So I would guess it would be more important when the case has lots of powder and little air?

 

[rcmodel]

Air space actually has nothing to do with anything, as long as you have the correct weight of the recommended powder in the case.

Reloading manuals will give you how much powder to use, and in some cases, a compressed load will have no air in it at all.

rc

 

[1SOW]

For instance I would guess that 124gr JHP from different manufacturers when seated to get the same OAL would result in different seating depth and resulting case volumes.

Yes.

If you know the length of the first bullet and the length of the second bullet, you then know the "change" in seating depth using the same cartridge OAL.

Example:
PD 124fmj bullet at 1.13" oal.

XX 124 FMJ bullet (.03" longer than PD) at 1.13" oal.

The XX bullet will be seated .03" deeper in the case than the PD bullet. This is a significant change in 9mm.

Use the manuals if available.

Different Bullets of the same type, weight and length still may show pressure changes, but it shouldn't be a big issue if loading light.

I just switched from PD 124FMJ to MG 124 FMJ using the same powder and OAL. They are virtually the same length and there was very little change in performance.