Stick w/ sabots in a rifled barrel. assuming your scope/mounts and barrel nut are tight, make sure you have a steady front, AND rear rest, along w/ a solid bench. A sand bag on a tailgate ,aint gonna cut it.(not saying you did that) MAKE SURE YOU DONT REST THE PUMP ON THE FRONT REST, you will not likely get consistent groups! rest the mag/receiver on the front bag and pull the buttstock back tight into your shoulder, hold the forend downward.

Forget about the wizzbang 50cal. bullet sabots, use one of the chunkier slug/thinner sabot slugs. IME they are more accurate/ consistant in most guns. I recommend:
-lightfield hybrids 2.75
-hastings laser mag
-brennekke KO sabots


As far as breneke and foster style(rem sluggers) go, they are designed to be squeezed down through a choked barrel. remington actually use to choke their rifle sight smoothbore barrels to IC. Thats what the rifleing (grooves) in the slug are for, the slug does not actually spin. Ive looked down the barrel of a smoothbore that Ive fired slugs through, and saw no twist. Ive tried "rifled" slugs in a rifled barrel and got poor results, however you can put a cleaning rod and a copper bore brush, in a drill and get almost all of the lead out, you can also try black coated brenekee rifled slugs to cut down on leading I think brenekee even recomends them for rifled barrels and smoothys. I tried them in a rifled barrel H&R trackerII and a ithaca deerslayer II did get less lead in the barrel but accuracy was poor but mabye theyll do better in your gun.