I recently had the same issue with a model 13-2 that had a barrel change and because it was "clocked" slightly to the right (as you're looking at the sight as you're shooting) it shot about 3" left at 50'. I took it back to my smith and had him torque it more which centered the front sight, it now shoots fine. As it turns out, a few degrees off center make a huge difference at distances.

Brian