I dont have exactly the same set-up (mine is an 870) but I've tried several different slugs and I found that the Winchester 2 3/4 inch sabots work best for me. My groups are easily in the "pie-plate" at 100 yards (have not tried further than that). I didn't do as well with 3" shells...but that might be because I was seriously dreading it everytime I pulled the trigger
Good luck