"dark bore" - could be soot fouling; could be rust pits. In my bore scope, I have seen both - sometimes at same time - in elderly (WWI and older) rifles. I have tried WipeOut, steel wool, various solvents - can see in bore scope that crap goes away, but pits make the grooves still look dark - no way that I know of to restore that to "new" looking - but is no rust or fouling left in there - patches come out more or less "clean" (now - after multiple treatments with various stuff), but still "dark" bore.