You want rifled slugs. Lots of companies make them; I'd buy a box each of several different ones and take them to the range to see how they shoot out of your gun.
Brennake says that modified or improved choke is best. Most guys say to stay away from a full choke but the truth is a rifled slug is designed to fit through anything but extra full. It does work better with modified or improved, though -- I find improved is even better than skeet or full bore. It kind of depends on the round and your individual shotgun.
I got a passive fiber optic sight to put next to the post on my Remington smooth barrel. It doesn't shine, but it does help aim a bit better. Other options include getting a barrel with rifle (iron) sights (they make them in both smooth and rifled barrel) or mounting a scope. I don't know about Mossburg, but Remington guns can get a scope mount that fits over the receiver. Use a low power scope that's designed for shotguns or black powder; the recoil will tear up a rimfire scope. (Although some people report using scopes designed for rimfire guns with great accuracy for a long time, so you can try if you want.)
Finally, range your gun. Most rifled slugs "rainbow"; mine shoots dead on at 25yds, 1" high at 50, dead on at 75, and about 1" low at 100.
I've moved to a rifled barrel and like the accuracy much better, but smoothbore is still an option.
ETA: I can shoot Remington Slugger rifled slugs for less than $1/shot, whereas my .270WSM is closer to $3/shot. So getting a box each (five to a box, so about $5 for the cheapest) isn't a bank breaker.
Last edited by Barefoot Friar; 11-12-2014 at 08:32 AM.