I have also encountered this. I believe the culprit was over tightening those barrels. I had a guy bring a new model Ruger Blackhawk into the shop that just would not shoot to please him.
It was a 45 Colt. When I tried to pass the range rod through from the muzzle, it would not go past the forcing cone. Pulled the barrel and installed another everything was fine. Again, over tightening was the culprit.
