Shooting 133 in a PPC, cleaning with a bronze brush, Butch's Bore Shine, and patches, I have not had carbon rings show up, but with other powders I have, and to remove them, I have CAREFULLY used IOSSO on a nylon brush, short stroked in the throat and the back 8-10 inches of the barrel, or a little farther up the barrel, with slower powders in larger calibers. A bore scope is very helpful when dealing with this. One does not want to use abrasives more than is necessary to get the job done, and good rod technique and a better than average bore guide, with a bushing on the rod, should be SOP.