Frank, most of us go through the process of going back to square one peg one on the examination of our cleaning process. Carbon build up is terrific in all of these pictures.
Brushes and abrasives, brushes and abrasives....brush is good for 100 strokes max, if you don't believe it, measure a new brush, then again after every 20 strokes.
On barrels like this, I put a cork in the end of the barrel and soak for two week intervals with a penetrating oil called Free All...best there is. Let the barrel stand up and sit for two weeks, pull the cork, then give 20 strokes with a good bronze bristle brush, dry patch out, re examine. Repeat the process. Usually three soakings will get it out, and a follow up with Montana Extreme Copper cream gets the tiny little molecules out that look like minute pepper that is left.
Good luck