I've used it for years on stubborn carbon and as rust loosened. It takes a while to get at harder carbon, so I switched to Slip2000 Carbon Killer for a really tough carbon job, like the inside of an M14 gas piston that has been sitting around for years without cleaning.
Gunzilla makes a copper-removing version, but I haven't tried it. I like KG-12 and Bore Tech Cu++ (also non-toxic, IIRC) are just fine for copper, but I use them before using Gunzilla to avoid the water and oil compatibility issue. So, I know of nothing against Gunzilla. Just be aware that it dries to leave a thin lubricating varnish on a surface. The maker says this proved to end stoppages in the war in Iraq for those who used it, so it isn't a bad thing; just a thing to be aware of so you don't create a barrier to your copper solvent if you aren't using Gunzilla's copper solvent version.