Wedgy, A penetrating oil called Free All is a Modern Product that has some chemical properties that can actually attack carbon, you will throw away your Kroil...no kidding.
Bore scope, where is is? You may have verified your process already, and I apologize if that is the case.
Bronze bristle brushes, buy in bulk, use 50 strokes, then use the brush with patch/JB or ? on it, throw away often. We need a vendor to sell bronze bristle brushes in 120 count boxes.
I use a product called Montana extreme Copper cream instead of JB, on patches, prior to using Patches with JB on used brushes...miracle stuff for tough R#15/XBR 8208/Varget fouling.
The issue with carbon fouling is having a constant amount. Carbon builds up on carbon fast, and it will get so hard that only significant abrasives will get it out.
Free All penetrating oil is a game changer with a good brush.
Beware of abrasives, they can open the throat dia and length of leade, use sparingly....again, sparingly. I hate surprises.
Context in cleaning is important, mine is 30 rounds in a 308 winchester, Krieger barrel. 8 rounds on a 6 ppc maybe dramatically less dramatic than my application.
Bottom line, Brushes are over used. Change brushes often, use lemon water or Kerosene. For plastic Brush users, check the Coefficient of Friction of plastic on steel in the machinist manual...get ready for a shock.