Originally Posted by CalTeacher
Perhaps. Ive never trimmed a single piece of pistol brass and don't know why anyone would.
because crimping is based off of the length of the case. cases differ up to .015" in 9x19 if i remember correctly. No way you get a consistent .002" crimp on all rounds. inconsistent crimp results in high SD and ES. in capable guns that causes accuracy loss.
of course plinkers, broadside of a barn and 7-yard shooters need not worry about trimming anything.
rimmed cases with roll crimp are especially sensitive to this if you're going for accuracy. I've chrono'ed my 38spl, 357mag and 44mag loads and uniformed cases make a difference when you're shooting for under 2" groups at 50 yards.
Cartridges head-space off of the mouth with light taper crimp is much less sensitive to case length. But if the crimp is too light, you'll end up with rounds with belled mouth that causes feeding problems. Ever wonder why reloaded ammo are so much less reliable? my trimmed cases have less ammo related hang ups and less 'fliers' in machine rest.