Re: Need Help
Most revolver cartridges have a protruding rim that sticks out beyond the body of the cartridge. The rim is what holds the cartridge from the blow of the firing pin.
Semi-auto handgun cartridges have no protruding rim. There is a rim but it is usually the same diameter as the case body. The grove that makes that rim is there for the extractor to grab in extraction of the case after firing. The lack of a rim is done to aid feeding from a magazine. Semi-auto cases need some way of resisting the blow of the firing pin. The chamber of the barrel has a tiny ledge that the cartridge case mouth seats against to resist the blow of the firing pin. But what does this have to do with "crimps".
Revolver cartridges can use a rolled over edge on the mouth of the case, called a crimp, to retain the bullet in the cartridge. Semi-autos need the mouth of the case straight so it will hit the ledge in the chamber. They use a taper crimp which leaves the case month straight but only ever so slightly pushed against the bullet. The difference in size of the bullet and the mouth of the case, as created by the reloading dies, creates what is called neck tension that retains the bullet in the case. The taper crimp is done to eliminate the belling done to aid bullet seating during reloading and to lightly push into the bullet.
Hope that explains the why of crimps.