That's what I'm trying to find out. I haven't found any actual reference to a chemical composition for the very thin layer that we see as irridescent shades of blues, purples, yellows, etc. I know for certain that the phosphoric acid rust removers will remove case colors. The ones that I've used are OK for exhaust manifolds and the like, but bite far too agressively for use on all but the nastiest barn gun.

Voting for anti-gun Democrats is dumber than giving treats to a dog that shits on a Persian Rug