Stone Cold,

There are several good reasons to crimp rifle ammo, just as there are good reasons not to crimp. There are three style of crimp. Rolled, tapered and star crimps are the most common.

Star crimp.
This crimp doesn't require a cannelure on the bullet. The crimp forces five different points in the neck of the case, near the mouth, into the bullet. I don't like this crimp and don't use it. This crimp requires a seperate die to form on the case.

Taper crimp.
This crimp also doesn't require a cannelure on the bullet. This allows you the seat the bullet to the over all length you want, and requires a seperate die to form. This crimp tapers the mouth of the case to a smaller diameter and tightens the grip of the case neck on the bullet. This die requires a deft touch to get the die adjusted to the correct length. If I were still shooting national matches with semi-auto rifles, this is the crimp that I would use. This crimp is also used on semi-auto pistol ammo.

Roll crimp.
This crimp requires that the bullet used have a cannelure into which the mouth of the case is rolled by the seating die. This doesnot allow for varied seating depths. The crimp can be performed at the same time the bullet is seated, but IMHO, I prefer to get a second seating die, remove the seating stem, and use it to form the crimp in a seperate operation. The die used to seat the bullet is adjusted so the crimp portion of the die doesn't come into use. This crimp is used on rifles with a tubular magazines, semi-auto rifles and revolver cartridges.

IMHO, all crimps require that cases, even pistol and revolver case, be trimmed to the same length so the crimp is consistent on all cartridges. With rifle cases, this means they should be trimmed to length every time they are sized. It is amazing how doing this to revolver cases will improve accuracy.

The reasons to crimp are many, these are just the most common. If you're using a case full of slow burning powder and the accuracy isn't what you think is should be, the problem may be due to poor ignition. A crimp will slow the release of the bullet from the case neck long enough to improve ignition and help accuracy. This problem is also demonstrated by a large velocity spread. The improved ignition will also reduce the spread. This problem is most common when ball powders are used, Ball powder are difficult to ignite. Very cold temperatures may also cause this problem.

Rifles with tubular magazines put a constant pressure against the nose of the bullet and may cause the bullet to seat deeper in the cause. This raises pressures to a point you really don't want to go to. I like the rolled crimp for this situation. I have never had a case that was roll crimped have the bullet move. That includes a 1895 Marlin in .45-70. I have intentionally marked a case with a felt tip marker and left that cartridge the first one in the magazine during a day of plinking. The lead nose was compressed, but the seating depth remained unchanged.

Ammunition used in a semi-auto firearm is subject to some rather harsh treatment. A crimp helps keep your ammo the way you made it. Having a bullet pounded deeper into the case may turn a nice rifle into junk.

A few of the reasons not to crimp are: Crimping takes time and effort. Not crimping is easier. IMHO, there is really only one valid reason not to crimp. What a crimp does to the case. A crimp works the case and the necks tend to harden and split. I do a lot of work prepping cases and don't want to lose them any sooner than necessary.

This is far from being all the reasons for/against crimping, these are just mine. I hope that this is of some help. Good luck. God Bless.