It's more complicated than it seems, especially if one doesn't really understand the physics involved, IMHO. As I see it, with regards to gun barrels, it's the specific relationship between the barrel's metal and the projectile. There's apparently an optimum smoothness of a bore that will have the least effect on a particular copper alloy, in terms of copper fouling. On one end of the spectrum, if the bore is too smooth, then you have an issue of the copper alloy tending to fuse with the very smooth bore surface as the smooth surface actually presents more contact surface than a rougher surface. At the other end, the bore can be rough enough where it acts on the projectile like a fine file, scraping off copper material. And different copper alloy's will react differently to different smooth surfaces.
It's been a very long time since I studied physics in college, so I'm probably not explaining it quite correctly . . . particularly with regard to the fusing aspet. But if you're really interested, you can probably do a google search and find some expert explanation of this phenomenon.