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.
Great post.
I would be willing to bet that load A was running way high to begin with and waaaay more than 20 rounds was fired. The primer cratering can happen at low or high pressures depending on headspace or other issues. It is not a reliable indicator.
I have fired 1000s of rounds through some guns I have without cleaning (ARs) and have not seen anything approaching the over pressure signs in A much less B in any of them. Ever with a lousy barrel and hot korean military ammo.
As far as handloads go, in competitions, I have fired hot handloads for 88 shot strings (not including the foulers before I got on the firing line.) Never have seen anything close to this!
I appreciate the info but real numbers would be of real help here.
what were the actual shot numbers and load?
P.s. This blog is excellent.
Thx for this. I actually had same issue during match. 6xc, relatively safe load 38.7gn (900ms) N203B. Max load is 40gn (950ms) @ 0C. Before match I had extensively clean barrel,using most of copper/carbon removers. During match (150 rounds, -2C) rifle started showing light pressure signs after 60 rounds. After 80 rounds every third casing was missing primer.
This is with relatively new barrel (385) round thru it. LW stainless barrel.
Some barrels build up less carbon/copper others more. But this is deffinatly something a reloader/competiteve shooter must be aware of.
That is my photograph [used without asking me] and the pressure as not caused by Copper fouling.
It was 308 brass necked down to 243. The tight necks caused bullet pinch.