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.