You can shoot a whole life with a 9x19 pistol without any problem, but try to do the same with a .222, and problems are much worse with the higher speed you get.

Certanly is the surface and fit of condereable imporance, but the speed should not be overlooked.

Regards H�kan

Bullet fit has more to with this than bullet speed. If your bore is oversize at the bore or elsewhere, hot gases bypassing the bullet melt and deposit copper in the barrel just forward of the overbore area. This is bad. The rifle should be rebarreled. If the copper is in the end of the barrel, this is more likely from friction and more normal, just don't let it build up or accuracy will suffer. It does not damage the gun, just keep it clean.

Bore surface roughness has a great deal to do with copper deposition, which is the main reason for all the fuss about barrel break-in techniques.

I really find it hard to believe that the gasses melt copper off the bullet jacket and deposit it to the bore. I'm pretty sure that the fouling comes from friction of jacket to bore, but I'm always willing to listen to a new theory. I think you are correct with lead bullets, but not so sure about gliding metal jacketed bullets.
One other factor that seems to allow barrels to foul quicker is to get them hot as in rapid fire.

regards,
Graycg

The used 9mm Parabellum pistol I recently bought came with the bore surfaces nearly completely orange-brown from tarnished copper fouling. So much so that I had to inspect it closely to be sure it wasn't rust before I bought it. Certainly it was as copper-fouled as any rifle barrel I've ever seen. I've seen a good bit of coppering of .357 and .44 Magnum revolver barrels, as well. The reason that copper fouling isn't a problem in these guns is that they're not expected to shot sub-1" groups at 200 yards. They have larger bore diameters to start with, the fouling has less effect on the bullet than the same thickness in a smaller bore, the fouling likely affects the bullet less when the velocity is lower (I'm speculating here), but mainly the accuracy of the gun is so much less to start with that the difference due to the fouling is unnoticeable.

Again, copper fouling occurs in the throats of rifle barrels, not just in the muzzle. Bullet velocity there is no higher than in pistols. However, chamber pressure is higher, therefore the acceleration "slugging up" the bullet, causing it to bulge and press the jacket against the wall is higher. Also, powder gas erosion roughens the surface of the barrel throat.