Loghead,
I have hunted quite a bit with shotguns, especially smoothbores. I have always found that each and every gun would shoot a different slug the best. What worked for one gun, almost never worked in another. Oddly enough, the cheapest slugs out there usually performed the best for me. My biggest problem that I found was when I shot slugs out of a full-choke barrel. The barrels that were modified or improved cylinder always shot better for me. I once took an old Stevens single-shot break action that had a 28" full choke barrel. I cut the barrell off with a hack saw, (I was only 17 years old, didn' t know any better ) I cut it down to 22" . I have no idea what the choke would have been on that barrel. It shot beautiful. At 50 yards I could group the slugs into 4" . Not that impressive but it only had a bead on the end for a sight.
I purchased a rifled slug barrel for my Mossberg 500, it has the cantilevered scope base welded to the barrel. With a 2.5 power scope, shooting Rem. 1 oz. " slugger"
slugs, it shoots into one ragged hole at 75 yards. (5 shots) Of course the ragged hole is about 2 and a half inches across.