Hi Bidyanus, I am a little confused as to your spread of shots. Have you slugged the bore all the way to the muzzle. I am no expert by any stretch but is there more wear at the muzzle for instance? Thus giving you a large spread. I would have thought each individual barrel would still group better than that even if you couldn't regulate them easily considering the difference in bore measurement.
Excuse me if my questions are to simplistic and you have already looked these issues I ask also for my own education and knowledge gain in these matters, I also hope you find a solution through shared knowledge on this site.