Try the "Truball" rifled slugs (I prefer the low recoil version). Excellent long range accuracy from smoothbore shotguns. Much better than their plain old slug. Really only limited by the sights. I shoot them in a 20" smoothbore with rifle-type sights, a bead sight was not able to get the most of their acuracy.
PB127LRS
http://www.federalpremium.com/produc...ug.aspx?id=395