I didn't slug my barrel but found that .312 lead bullets shot absolutely horrible from my 32H&R Cowboy. I moved up to .313 and they shoot acceptably well at 50 yards. .312 Hornady XTP's shoot great at 100 yards over H110 powder, usually 2" or less at 100 off the bench with some 5 shot groups going less than that. While that's not benchrest accuracy, it certainly is acceptable for me and works well on the occasional groundhog to 100 yards or so. Your slugged measurement sounds pretty big.
Good shooting.