He is probably talking about Thompson Center's coned breech. Think of a short ice cream cone and that is what the back of some barrels near the breech look like. I take a small 30 caliber brush from time to time and wrap a patch and reach back in there and scrub around. Strange, I never really find anything. I also use a breech scraper from time to time in the back of the sidelocks. I think a good hard water flush knocks 99.9% of the fowling out of the cone, but I am sure that is what he is refering to.