Are you serious? 3.5" slugs? Tear the shoulder right off ya! There goes accuracy........Lemme guess dude, you shoot a super magnum short, fat wonder round in centerfire to bag a 100 lb. whitetail?
O.k. Sarcasm over. Sorry. If you could find/load those shells, maybe a rifled choke? I've never seen a 3 1/2 " slug barrel advertised or catalogued in any of the places I looked before I got my 3" fully rifled pipe.