This is not an expert opinion, but I think it would work better to have a gunsmith duplicate the contours of the original smoothbore barrels with new fully riflled barrels, then remove the smoothbore barrels and replace them with the rifled barrels. Based on the way most slug guns recoil, I would consider starting with a 12-ga double and using fully rifle 20-ga barrels for the replacement. That way the greater thickness of the new barrels would add some weight for less recoil. Then I would use custom turned brass cases from Rocky Mountain Cartrige and cast heavy lead bullets for it. I did that for a Thompson-Center TCR87. That is, used a 20-ga fully rifled barrel with the outside conture of a 10-ga. It weighs about 8.5 pounds with scope and shoots 550-grain bullets at 1,330 fps with good accuracy. The bullets are cast out at 720 grains, but the weight is reduced to 550 grains by drilling out hollow points. I would guess it would be very expensive to regulate the barrels to hit to the same point of impact. However, by varying bullet weight (which can be changed by varying the depth of the hollow point) and varying powder charge, I think one could get regulation accomplished at no cost. However, the loading dies and bullet cast are not cheap.
