RE: max distance with a smoothbore shotgun
I have a remington wingmaster 12ga that will shoot 3" groups all day at 100 yards with several different brands of rifled slugs and an open choke tube. My Mossberg will only get those kind of groups at about 75 yards or so. Every shotgun is different, so you just have to get out and shoot several different kinds of slugs to find out what shoots best and how far you can expect to hit with your gun.