TexasCHLforum.com

The importance of crimping depends on what kind of gun you are loading for. If you are loading for a Glock, with a barrel which tips down during cycling, it is less critical.

If you don't crimp and you are loading for a semi-auto that has a fixed barrel, then, as the cartridge chambers, the bullet can be pushed back into the case as it hits the feed ramp. This causes higher pressures which can be critical.

I.E., makes a big kaboom instead of a bang.