You may not have understood the problem correctly. The ridge was steel that had copper built up on. It would take very, very careful hand lapping to remove it without significantly loosening the bore and muzzle diameter. But the muzzle diameter was already too big on another gauge, and nothing was going to heal that. It is also the case that such ridges can result from steel being deformed by ringing the barrel, which can happen if you run powders a bit too slow for the bullet weight, though I don't know that happened here. Interesting problem, though.I am still giggling over the idea of hacksawing off the end of the barrel, rather than finding a way and just cleaning the ridge out of the bore.
In any event, until a fertilizer that grows steel is invented, there's no more practical solution than removing the bad muzzle or at least counter-boring it. M.L. McPherson cuts barrels at the shooting bench when trying to tune them. He saws, then files with reference to a small square, then laps uniform with lapping compound and a large marble.