Crimping prevents the bullet from moving rearward into the cartridge case during recoil or any other action that may cause the bullet to move. If the bullet moves too far out of spec (either way), dangerous pressure may result during firing.

In terms of accuracy, I personally get the best results from uncrimped loads. I don' t crimp ANY of my bolt action rifle loads, for target or hunting purposes, but I do put a mild taper crimp onto all 308 Winchester I load for my brother' s semi-auto.

Anything going into a tubular magazine MUST be crimped.