Crimping... when and how
Bullets need to be retained in a cartridge for several reasons. The obvious reason is for handling and transportation. But the bullets have to remain in the case during the initial stages of the ignition of the powder so that the pressures are uniform from shot to shot. In recent years reloading dies have been made to have more case tension on the bullet than in years past but some calibers used in some types of guns need a crimp too.
Guns that treat the ammunition in the gun violently, through heavy recoil or rough handling during chambering, need a good crimp. Guns that hold the ammo in a loading tube in the gun, end to end, need a good crimp. Typically all semi-auto guns, most lever guns, and pump guns need a crimp. There may be others but the test is the above where the results of poor bullet retention can end up with either the bullet falling out of the cartridge case in the gun or being pushed into the case. While both situations are not good, the latter can be dangerous as the pressures rise as the receding bullet reduces the volume in the case. It is a definite safety issue with either case. It never hurts to crimp a bullet into a cartridge case but it has to be done the correct way.
In general there are three common crimp types used today: Roll, taper, and LEE Factory Crimp:
The roll crimp is a rolling of the case mouth into a crimp groove or a canalure in the bullet. The edge of the case literally rolls into the bullet to help retain the bullet. It is done most commonly by the seating die but can be done in a separate operation by a separate crimp die. The case mouth is pushed up against a shoulder in the seating die and it rolls over to varying degrees dependent on how the die is adjusted. If done too much it will bulge the mouth of the case, making the cartridge so large in diameter at that point that the cartridge will not enter a normal chamber. Cartridges that headspace (the place on the cartridge that stops the cartridge on its path into the chamber) on the rim of the cartridge, on the belt of the cartridge, or on the shoulder of the cartridge can use a roll crimp. Typical examples are: 38 and 357 Magnum, 41 Mag, 44 Mag, 45 Long Colt. Note that most are revolver cartridges but all use the rim to headspace on. Typical rifle cartridges would be 30-30, or any rifle cartridge that headspaces on the case shoulder. Lever guns are prime candidates for roll crimps. Cartridges for semi-auto guns, like the 223 used in the AR-15, are also good candidates for crimping due to the violet nature of ammo handling by semi-auto guns. Note that the case lengths must be the same on a batch being reloaded for the roll crimp to work correctly.
The taper crimp is where the case is pushed into the body of the bullet so tightly that it secures the bullet from moving. The case mouth is pushed by a tapered surface in the seating die or a separate crimp die into the bulletís body. It can be applied to varying degrees dependent on how the die is adjusted but there must still be an exposed edge of the case standing proud of the bulletís body. That edge is what the case headspaces on because in most of these cases there is no shoulder or rim on the case. Typical examples are all the semi-auto pistol calibers like 9mm and 45ACP, among many others.
There are two entirely different version of the LEE Factory Crimp. The first is the rifle version. It uses a collet in the die, actuated by the case itself, to squeeze horizontally at about four places around the mouth of the case. Since the squeeze is horizontal, the case length is not all that important. This crimp emulates the one seen most often on commercially loaded ammo. If over done it will distort the bullet and that may affect the accuracy. The rifle version of the LEE factory Crimp is a better crimp to use for rifle rounds then the roll crimp.
The other LEE Factory Crimp Die is for handgun cartridges. It is a separate crimp die that makes a crimp similar to the roll crimp for revolver cartridges and a taper crimp for semi-auto cartridges. It includes a carbide ring in the bottom of the die that sizes the outside of the case. It acts as a cartridge gage to assure all reloaded ammo will fit any and all guns of that caliber regardless that earlier processes caused some distortion of the outside of the case