Roll crimp?

The crimp is rounded into the bullet grooves instead of being a straight taper when a taper crimp die is used. Typically, a roll-crimp is used for heavy-recoil cartridges so the bullet doesn't shift, as it is more-secure than taper crimps.

It should never be used on cases that headspace on the case mouth (e.g. .30 Carbine, .45ACP).
 
Handloading dies come with the correct type of crimp for the particular cartridge. 9MM, .45ACP, etc. will come with a taper crimp. .38 Special, .44 Magnum, .45 Colt, etc., will come with a roll crimp shoulder in the die.
 
With a roll crimp, the case mouth hits a shoulder in the die and is bent/folded (rolled) inward against the bullet.

With a taper crimp, the die tapes, gradually squeezing the case against the bullet.

A roll crimp works best when there is someplace on the bullet for the brass to go. (crimp groove or cannelure) A taper crimp works with bullets that don't have that.

In the context of straight walled handgun brass, most of them are fired from revolvers, and a crimp is often needed, because in a revolver recoil acts as an inertia bullet puller (sort of in reverse), and a roll crimp is strongest.

There are combinations of gun weight, bullet weight and recoil force that will defeat a taper crimp. This is also possible with a roll crimp, but much less likely.

Roll crimp also does a better job of standing up to the pressures of a tubular magazine, and is a virtual necessity for that kind of use.

Also, a (heavy) roll crimp's hold on the bullet aids in combustion of slow burning powders used in magnum pistol loads.
 
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