Unusual barrel fouling
I have had good results cleaning old and neglected firearms found at gun shows and pawn shops. This one has me at wit's end.
Remington 510 Targetmaster, .22 caliber, 1942 vintage. After cleaning, I found the walnut stock and all metal parts to be in very good original condition. The exception is an unknown crud deposit about 7 inches long measured from the muzzle. Grooves are filled and lands in this portion of the are barely visible. Hoppes #9, Kroil, and Lead Remover have been tried. The barrel was soaked overnight in each of these solutions, brushed with a phosphor bronze brush, and swabbed with patches. Black residue on the brush and patches. Tried to scrape the mystery fouling from the grooves nearest the muzzle with a steel probe but nothing was removed. So far the lands are becoming a bit more pronounced but the going is very slow.
Any educated or experienced guesses about what this fouling is and how it can be removed?