I got a Nazi marked Mauser from Southern Ohio Guns, and it had the dirtiest bore I've ever seen. It's still not really clean, even after 8 or 10 passes through it with super solvent.
I'm going to get one of those electronic bore rods one of these days . . .
and buy a can of "Gun Scrubber". It'll dissolve that without a problem. One warning though, DO NOT use it on polymer framed guns because it'll leave a nice mark where it started to dissolve the plastic.