I am guessing that the deposits are copper.  I have only shot jacketed rounds in it. Although it's possible that there is cupro nickle fouling in the bore. I thought I was doing a good job cleaning it. As ugly as that bore is it usually groups around 3" at 100 yards with iron sights and my eyes. For me that's pretty good. I use hoppes no. 9 for cleaning. I will try to locate some kroil. It sounds like the ticket for copper fouling.