Barrel regulation is likely the problem. Ran into the same issue with a .30-30/20 Mod 24. The gun had not been modified and the rifle shot quite well. #3 buck pattern center was about 24" high at 10 yards. Down the road it went.

I am..........disturbed.

Concerning the difference between man and the jackass: some observers hold that there isn't any. But this wrongs the jackass. -Twain