[ashercg]

I know that straight wall pistol cartridges headspace on the case mouth (I load 45acp, 9mm, 40 S&W and 380). Case trimming length recommendations (reload manuals) and detailed measurements of my pistols lead me to believe that it is up to the firing pin to seat the case mouth when firing (except the PPKS 380 with it's blowback action which, according to my measurements, is seated by the slide spring). All locking actions have slack (tolerance/slop) when locked with no cartridge or recoil spring involved.

Would a more accurate round result from trimming my cases to fall within that slack so that the slide spring seats the case mouth? I would trim to the short side of the slack range to allow for soot/dirt buildup (I thoroughly clean this area and the lock engagement areas as part of my cleaning routine after every range session)

I think that my 9mm cases are long enough to do this. My 45s seem way to short.

[Otto]

No need to trim 45ACP unless it's longer than .898" (which is almost never).
Here's an article that explains why...

http://findarticles.com/p/articles/mi_m0BQY/is_1_48/ai_80635898

[243winxb]

The longer the case, less end play in the chamber. Better accuracy. If i would need to trim, i would not go shorter than the maximum trim length as there is to much headspace in most chambers. BUT you should measure your chamber before doing this, know your gun well. The 45acp will never need trimming. But to be safe, check the length before loading.

[NuJudge]

My experience has been that all your rimless pistol cartridges get shorter with use.

CDD

[jmorris]

I case gauge every round I shoot in competition but have never trimmed any non-bottle neck case.

[ar10]

Working at a range I come across a bunch of 45APC brass. Problem is I don't have a .45 and will likely never own one. I do have a .45GAP and got the "bright idea" of taking a .45APC and trimming it down to GAP specs. After all they are the same in every other way.

WRONG :what:
The difference is pretty big. The GAP is designed using +P powder charge and has a thicker base than the standard APC round.

[Walkalong]

I trim revolver brass one time after its first firing and size them prior to that trimming. I never trim auto brass, and have never had a problem. :)

[DEDON45]

I've never trimmed pistol brass, nor have I seen (or measured) a reason to do so... the cases often fail (split, etc.) before that becomes an issue, or they are lost during a range session.

[Vern Humphrey]

My experience has been that all your rimless pistol cartridges get shorter with use.

That's been my experience, too. I've reloaded literally hundreds of thousands of .45 ACP rounds, and never trimmed a case.