Some time back I raised the topic of crimping as part of a discussion on reloading dies, and promised to address the issue more fully at a later date—which is now.
What is Crimping?
During reloading, cases are resized and the case mouth is expanded (belled) to accommodate a new projectile. Crimping is the final task which presses the case mouth back around the projectile. Crimping can be of two basic types:
- Taper crimp
- Roll crimp
Figure 1 shows a Taper Crimp and Figure 2 a Roll Crimp. The difference is barely noticeable. If you look closely you will note that in the Taper Crimp, the case mouth is somewhat flatter against the projectile, whereas the Roll Crimp actually rolls the case mouth in a curve onto the projectile, i.e., the Roll Crimp tends to be more aggressive.
Factory ammunition will come with a factory crimp, which is specific to the manufacturer, but will be one of, or a variation of the two basic crimp types.
Some reloading die manufacturers may also have their own unique crimp styles, e.g., Dillon Precision’s “Accu-crimp” revolver crimp die.
The general consensus is that taper crimps should be used for semi-autos that use rimless cartridges and rolls crimps for revolvers with rimmed cartridges. However, what crimp do you use for a semi-auto that uses a rimmed cartridge such as a Desert Eagle .357/.44/.50, or a S&W 929 revolver that uses a 9 mm rimless cartridge? More on this issue later. The debate abounds.
Why Do We Crimp?
There are two major reasons for crimping.
Firstly, a crimp holds the projectile firmly in place during storage and transport, ensuring the round remains in the same physical condition that it was in at manufacture/reloading. This preserves the physical integrity for loading, feeding etc, and maximizes the chance of the round achieving the power factor it was designed to produce.
Secondly, during firing, a crimp increases the friction between the case and the projectile ensuring the projectile does not leave the case until all the powder charge has been ignited. Assuming the powder charge is correct, un-burnt powder in the barrel is one sign that the crimp may be too light.
Is Crimping Important?
In a word, maybe; it depends very much on the type of case and projectile. There is one school of thought that advocates crimping only if absolutely necessary. However, for most pistol ammunition, which is what I’m focusing on, this is generally the case. Some long projectiles may have sufficient friction between themselves and the case as to not require crimping. In this case, a very mild crimp action to simply remove the case mouth belling after bullet seating may be all that is necessary, i.e., make the case mouth flat against the projectile.
However, without a crimp there is a possibility that the projectile could move either further into, or further out of the case. This may happen during firing, during transport or if the ammunition is (accidentally) dropped. If you have ever used an impact type bullet puller, you will have an idea of how little force is actually needed to move a projectile in a case, e.g., the recoil of a revolver, in particular large caliber revolvers, has the potential to actually move the projectile of a round which is in the cylinder, but not actually being fired, slightly out of its case during firing.
If a projectile has moved out of the case, the Overall Ammunition Length (OAL) will increase, which will reduce case pressure and the ammunition’s power factor. This may produce misfeeds in semi-autos. An increase in OAL also has the potential to cause ammunition to jam in semi-auto magazines, or stop the cylinder of a revolver from rotating
Conversely, if the projectile moves further into the case, the gap between the powder and the base of the projectile is reduced, which will increase the pressure inside the case during firing. In the extreme, the projectile may actually start to compress the powder charge creating a “compressed load”. This can produce extreme case pressures and can be dangerous.
In the next article I’m going to continue the discussion on crimping to cover additional issues specific to crimping for revolvers and semi-autos.