Quote Originally Posted by pa_guns View Post
You should not put a crimp on either one. You can get away with it on most 45's but it's a definate no-no on a 9mm.
High pressure rounds like the 40 and 9x19 absolutely, 100%, without a doubt require a crimp. When an uncrimped round hits the feed ramp, the bullet can actually get pushed back in the case. This will cause your pressures will skyrocket causing something along the lines of a Kb at the worst and popped primers or blown cases.

When reloading, you have bell the case (slightly more bell for lead bullets) to allow a bullet to go in, you also have to get rid of that bell so it will feed. You want to crimp enough to hold the bullet in place but no so much to hurt accuracy.

Semi-auto rounds get taper crimped and revolver rounds normally get a roll crimp. Two different dies.