In my experience with both, IOSSO cuts better than JB for hard carbon. Don't stroke the whole barrel just short stroke the affected area. The carbon is pulling the copper. You will also have alternating layers, so it is good to alternate IOSSO with a strong copper solvent in a case like yours, dissolving the copper as you uncover it. Working with a good bore guide, being careful with the rod, there is no virtue in making slow strokes with an abrasive. Laying down copper is exacerbated by bullet speed so there is a lot less concern about over polishing back in the throat than farther up the bore, and you can't over polish the steel when it is still covered with something else. Some years back, a friend was dealing with a very slight hard carbon ring at the back edge of the freebore, I had told him to use IOSSO but being hard headed he first tried JB on a patch scrubbing the problem area. It did not work, but with IOSSO he was successful. The whole thing would have been over and done with the first time if he had used the product and method that I suggested, but he was concerned about moving the throat forward. Many years later, a friend of both of us was dealing with a similar problem and he had been pussyfooting around because he had been listening to the first fellow. Luckily he had very carefully measured his throat. After failing with all other methods, he finally followed my advice, and the carbon was removed. After that he carefully remeasured his barrels throat, and it had not moved at all. It seems that my other friend had hamstrung his own cleaning procedures for years because of unfounded speculation. The second fellow who finally used the method that I advised did so with accurate barrel on a light varmint 6PPC that had developed some accuracy issues because of the carbon. After it was removed, the rifle's accuracy was restored. None of the problems that I have mentioned were even close to as bad as yours. One has a bore scope and the other lives close enough to use it. I have had one for years.
