I have on occassion just poured boiling water on a rusted item, including shotgun barrels, to convert brown to black. No reason it shouldn't work on a rifle, you could even pre-heat the barrel. I have a rifle to do and I intend to just pour boiling water on it. Give it a go, whats to loose, use a piece of scrap barrel. I think that's what you are asking, not about the rusting itself. --- John Can.