Crimping rifle ammunition

When it comes to crimping a bullet in place, it is usually something that is only done on pistol-calibre ammunition as the very final stage of the reloading process. In the case of rifle ammunition, reloaders generally rely on neck tension to hold their bullets in place and do not apply any crimp. However, it can be beneficial and should not be dismissed outright. Sometimes crimping is actually needed, but it should be completely avoided in certain situations. It can, however, often be a worthwhile option to explore.

Types of crimp
Basic reloading techniques normally call for a straight-walled cartridge to have a roll crimp, which simply curls the edge of the case mouth into the bullet to prevent it from moving back deeper into the case under recoil. In order for a roll crimp to work, it requires a bullet with a cannelure or a crimp groove, for the case mouth to be rolled into. Without one, you risk damaging the bullet and bulging the case, and this will have a detrimental effect on the finished ammunition.
When using bullets without a cannelure, it is necessary to use a taper crimp die, which squeezes the neck of the case inwards against the shank of the bullet. This tightening-down of the diameter of the case neck effectively increases its grip on the bullet.

When a crimp is needed
While most bottlenecked cartridges do not need crimping, there are a few exceptions to this rule. Ammunition used in tubular magazines is subjected to nose-to-tail compressive pressure when seated in the magazine, and this is increased significantly when the gun is under recoil. A good crimp will ensure that the bullets are not pushed back into the case when the gun is fired.
Many of the big cartridges, like the .500 Jeffery, are particularly hard-kicking, and the ammunition in the magazine really needs a heavy roll crimp to keep the bullets in place during the aggressive recoil. If the crimp is not sufficient, then the recoil can drive the bullet deeper and deeper into the case with each shot, and when chambered, that cartridge will generate much higher, and potentially dangerous, chamber pressures. Bullets for these heavy-hitting calibres tend to have cannelures to ensure that the roll crimp is effective and to avoid potential damage to the bullet.

When not to crimp
Applying a crimp to ammunition increases the hold that the case has on the bullet, significantly increasing the chamber pressure needed to overcome that stronger grip. If you are using ‘hot’ loads that are already near the maximum chamber pressure for the given calibre, you should not experiment with adding a crimp. This could exceed the safe chamber pressure limit, posing a risk of damage to both the firearm and the shooter.
Attempting to crimp ‘loose’ cases around small-for-calibre bullets usually does not work, and no matter how much crimp you apply, you will not get any grip. The application of excessive crimp can collapse the cases in the die and/or squeeze softer bullets where the case mouth is pressed into the bullet, causing a ‘waist’ to be formed around it. This actually results in looser bullets.
Heavily roll crimping bullets that do not have a cannelure can actually damage them. The crimping die will attempt to curl the edge of the case mouth into a space that is not there, and the brass will, if enough force is applied to it by the die, press into the bullet and cause a groove. Above this groove, a small bulge can be formed, and this will cause problems with chambering the round.

Crimping benefits
Crimping rifle ammunition can significantly improve rifle accuracy, and this is why it is well worth experimenting with. It is used to even out velocity, and it achieves this by increasing chamber pressure to a more consistent level before it releases the bullet. The theory is that the additional pressure that builds up, to overcome the crimp and move the bullet forwards, helps to generate a repeatable pressure curve, more consistent muzzle velocities, and improved accuracy. If the bullets you are using have a cannelure, then you can use a roll crimp. Otherwise, a taper crimp should be used.
The position of the cannelure governs how deep the bullet sits inside the case mouth and so dictates the Cartridge’s Overall Length (COL), and this may affect how your rounds chamber from the magazine and also where the bullet sits in relation to the rifling. If the COL is too long, the bullet can lodge in the rifling when it is chambered, and this can cause a spike in chamber pressure. When you use a bullet with a cannelure, you need to ensure that it does not make your ammunition too long.
If your bullets do not have a cannelure, then you should use a taper crimp die, as this does not dictate the COL, leaving you to experiment with different lengths.

Trial and error
As with other stages in reloading, experimentation can be the key to success, and crimping is no exception. The amount of crimp should be worked up from a safe and relatively low starting point to avoid large and dangerous chamber pressure increases. As you increase the amount of crimp, you will increase the chamber pressure, and you need to watch for signs of excessive levels. Once you have your crimp die set up, you can adjust it by just ½ a turn at a time and increase the amount of crimp you apply gradually, making small batches of ammunition with each degree of crimp.
Hopefully, consistency and accuracy will show signs of improvement, and with a bit of experimentation, you should be able to find a sweet spot where the amount of crimp optimises accuracy. If you start with your favourite pet load and apply a crimp, you may well experience an unexpected improvement.