Exactly what you are looking for is in the volume fourteen, issue 1, spring 2003, on page 29, of The Double Gun Journal ! It is an article titled Twelve Bore With A Twist . It Greener 12 ga shotgun that has had a pair of Hastings fully rifled, 1 in 36" twist, 12 bore barrels sleaved into the old barrel breech end. The shotgun was an origenal 12 bore with damascus barrels. Quarter rib, and Iron sights were added. Also a pair of Warne QD scope bases were added to the quarter rib, and a 3x Weaver scope added. The tubes are quite thin at the muzzles, and require very carefully heated tubes for regulating. This is best done with bore size externally heated rods placed in the barrels, so they are heated all the way around the tube to avoid warping the tubes. this method acentually turns the barrels themselves into soldering irons! The rifled 12 bore in this article was very well done, and I think, is exactly what you want to do!