Barrel Fouling and Pressure

Experienced handloaders consider many factors in determining loads for their rifles: type and amount of powder, primer “hotness”, brass strength in the web area, case neck tension, bullet weight, diameter and bearing surface, freebore length, seating depth, barrel land/groove configuration, and ambient temperature. Some folks even compensate for the altitude and humidity at the range. That pretty much covers it right? Wrong. You can get in BIG Trouble if you don’t account for barrel fouling as well.

Barrel fouling is not often discussed in reloading articles. Yet barrel fouling (both carbon and copper) can dramatically increase pressure inside a bore. It can do this in many ways. First a build-up of carbon ahead of the throat can increase pressures by constricting the bore. Likewise, heavy copper build-up can constrict the bore over a significant amount of its length. This means the bullet is being driven through what is, effectively, a smaller hole. As well, even a light layer of copper can increase friction. Added friction means more heat, and heat and pressure are directly related.

If you have a barrel that fouls heavily, and you need to shoot 20+ rounds with no opportunity to remove the fouling (say during a match), you should adjust your loads down so they are safe when the barrel is fouled. Don’t assume that a load which is safe in a “squeaky clean” bore will remain safe as your bore gets heavily fouled. This is especially true with non-lapped factory barrels with heavy “chatter marks” from machining. Look at this photo, provided by Forum member Clark. It shows the SAME load (identical bullet and powder charge) fired in the same barrel, at progressively heavier levels of copper fouling:

Pretty scary, right? Cartridge ‘C’, fired in a heavily coppered barrel, shows signs of catastrophic case failure, as well as damage to the primer. By comparison, cartridge ‘A’ merely shows some cratering, but the primer pocket edges are still rounded. ‘A’ is probably at the upper limit of a safe load. ‘C’ is at truly dangerous overpressure–with the same amount of powder in the case. The only difference was the amount of fouling in the barrel–particularly heavy copper.