I had a Garand once that had an exceptionally dirty bore. Bad enough that I didn't even want to try to shoot it until I had cleaned it properly.
As in your case, after cleaning it for varying periods over the course of several days, It was still returning astoundingly blackened AND rough patches. Finally, I got out my Hawkeye bore scope and had a closer look.
The entire bore had the finest and most evenly spread pitting from corrosive ammo that I had ever seen. It was so fine that even with a magnifying glass, it was not obvious at the muzzle end.
I hammered at it aggressively with a tornado brush - which I only EVER use as a last resort - and once determined it safe & sound, I took it out shooting.
It shot very, very well and 168 gr AP best of all. Too, after shooting it, while the bore was certainly not cleaner to any great extent, it sure did look better to the naked eye and the patches were less torn and blackened. So, between the tornado brush and finally shooting it, some of the edges of the 1,000s of corrosion craters within must have been somewhat smoothed off.
After I was content with that degree of shooting and follow-on cleaning, I tended to liberally apply RIG to the bore before storing it back up as each of those 1,000s of pits will forever be an open wound in the bore.
Todd.