I have heard it will remove lead,plastic and ground in carbon and powder residue.I used it in an old 30-30 that had never been cleaned and it did very well.
I loaned my only bottle out to a friend and he loaned to one of his friends and so it's all gone.
Might be worth a try.I would bet as someone else said its most likely lead.
Had one so badly leaded that I plugged the muzzle and poured Kriol in the barrel till it was running out and let it sit for a whole weekend and on Monday evening I ran a brush through it and man it all came out.Not totally spotless but way better
I have a weapon steam cleaner that will absolutely clean that barrel, in short order. Copper, lead, hard carbon...it all comes out. Hard to believe, but it really works. If you want to send it to me, I will clean it for you, no charge. Yours won't be the first "impossible" barrel I have resurrected.