To Crimp or not to Crimp

[JohnJak]

I see some ammo has some crimping and some do not. What's the purpose of the crimp? Does it help with bullet set back?

[JDG]

I only crimped 357mag when I had one. The factory crimp die I use for 9mm and 45acp, really don’t do much more than iron out the flare. Never had a setback issue ever The crimp on a 357, is more for keeping the bullet from unseating under heavy recoil. The taper crimp in a semi auto is for setback.

[joepistol]

Yes.. Magnum revolver cartridges, are most likely to experience bullet "set back" when in the cylinder of a revolver.
Revolver rds. usually have a crimp groove, or knurling ring on the bullet, and a "roll" crimp is recommended, to avoid bullet set-back,
and resulting dangerous pressure spikes from the set-back.

Because auto loaders usually headspace on the rim of the cartridge, a taper crimp is the preferred method of crimping bullets in this application.

[Ol` Joe]

I’ve seen the bullet set back into the case in semi autos during charging once or twice. Matter of fact I was talking to a fellow shooter about a round rotated in my magazine during my every 3/4 week cleanings of my carry gun that failed to chamber and had the bullet pushed back about 0.150”+. This was a factory 9mm of a brand I have been 100% happy with in the past that had been chamber no more then 2X-3Xs prior to failure.
Bullets with a crimp groove take a roll crimp intended to keep the bullet from backing out of the case during recoil as Joe already discussed and, are normally found in heavy recoiling cartridges Bullets without a groove take a tapered crimp intended to keep the bullet from setting back into the case during feeding in semi autos. These are the norm and not 100%.
The set backs I have seen have been almost always the result of a round failing to feed and the nose of the bullet jamming against the feed ramp or barrel hood and being hammered back.
The only cast of a bullet moving in the case was in a 45 LC revolver useing stout reloads of my making with long heavy for caliber lead bullets.
I learned a thing as to why heavy crimps that day, especially in shorter cylinder revolvers