Crimp

From GunCounter

A stage of the reloading process where the neck of the case is forced into a slightly smaller diameter to secure the bullet in place. This operation can be performed using the bullet seating die, or with a dedicated crimping die. Crimping as a separate operation will make loading on a single stage press slightly more time consuming, but many believe that it results in more consistent ammunition.

There are two types of crimp which may be applied, either a taper crimp or a roll crimp. Many bullets incorporate a special circumferential groove called a cannellure, which can enhance the holding power of the crimp. Care must be taken when adjusting the crimp die, because too light of a crimp can cause the seating depth to change during handling or under recoil, which can cause problems with consistent performance, or dangerous pressures. Too heavy of a crimp can also lead to dangerous pressure levels upon firing.

Not done with all types of rounds or all types of bullets.