Chuck,
I was with Don when he did some of the testing for barrel regulation on his Rizzini O/U 20 gauge. I don't believe gun fit has anything to do w/ the the barrel regulation. The same sight picture was used w/ each shot. Points of impact being high/low, right/left, for both barrels would be indicative of Poor gun fit, but not one barrel relative to the other given the same sight picture is used for firing both barrels.