It could very well be the gun, Wally. In 1998 I had a brand-new 5 1/2" .45 Blackhawk with two chambers bored not only off center-but, off-axis as well. It gave 14" 100 yard groups from ammo that had proved capable of 4", in a duplicate, non-screwed-up 5 1/2" Blackhawk.
I would go all over that cylinder with a good set of micrometers before I damned multiple bullets. If it checks out OK, have your cylinder throats reamed to a uniform 0.4525" and I'll bet your groups with all your bullets will shrink and round up nicely.
“Nine-tenths of tactics are certain, and taught in books: but the irrational tenth is like the kingfisher flashing across the pool, and that is the test of generals. It can only be ensured by instinct, sharpened by thought practicing the stroke so often that at the crisis it is as natural as a reflex.” ~ T. E. Lawrence