Martin,
A good friend of mine made a set of full length rifled barrel liners for a 12 ga dbl bbl shotgun, to regulate the poi he made off center threaded bushings for each barrel installed at the muzzle with a locking nut all you had to do was turn the bushing to get the barrel where you wanted it and lock it with the threaded ring, it worked well and was able to only have one set of sights on the gun.
Regards, Steve Chapman