Check the crown on the muzzle especially the RH bbl. Perhaps it has some damage or has been repaired from damage,isn't quite square, etc.
Also check the rifling at the muzzle in the RH bbl.See if it's been worn on one side.


Set the bbls up on a sturdy rest, with the sights right on a target dot at say 100yds.
Thensight thru the bores of each bbl. Just to see if perhaps the RH bbl is way off the mark.

I realize the bore allaignment won't be on the target mark, but it would seem if the RH bbl was set way off somewhere it might be the problem and not the load.