That ring or crimp around a bullet is called a Cannelure. In muzzleloading it really makes no difference to us as we are not crimping. I was always told that when we set a bullet into a cartridge case, that cannelure help to keep the bullet in the case, at a set place, and to seal in gas so that there is a uniform expansion when fired. That gives the shooter better accuracy in some cases. That's how it was explained to me. In a muzzleloader, placed in a sabot, that cannelure will do very little.

Many of the handgun bullets we shoot might have a cannelure. I have never seen a huge difference in accuracy.