With a cylinder bore it's pretty much wide open. (No pun intended). Any of the Forester El-Cheapo $4.00 for a box of 5 will work well out to 50 yards in my Mossberg 500 with an 18" cylinder bore. You can also fire the more high priced solid Copper Sabot rounds if you choose to. A lot of them are meant for rifled barrels and or chokes, but I've gotten the "minute of pie plate" results easily, many times much better. The best thing to do is buy a 5 round box of several, then set up a big enough target at 50 yards and put the biggest "Shoot-N-C" stick 'em target you can find on it, then have at it. Aim dead center with all of them and see where they track.
Slugs are a bit funny that way. I don't know if it's the varying degree of rotational spin they seem to get subjected to from different barrels or what, but many will print differently. In many cases I've found the cheap Forester hollow base models shoot a bit truer in this regard. Back in the 70's Forester slugs was about all that was avaliable except for the Rottweil Brenneke slugs that had those "rifled" wads attached to them. That was big stuff back then. They were loaded into transparent cases and you could see the slug, wad, along with the powder charge. Back in Illinois during shotgun slug deer season you were considered a real "Magaffer", (guy who had to have the latest of everything), if you showed up in camp with Rottweil slugs in your gun. One slug I haven't tried are the Remington Solid Copper Sabot models. I forget the trade name Remington has put on them, but they are big bucks, and everything I've read on them has been good. Especially for defensive shooting. I would never use slugs for that purpose simply because out here the houses take nothing to penetrate. Bill T.