Hi Big_Buck_Archer,

Any gauge from 20 or bigger is suitable. And particularly in 12 gauge any cartridge length is fine. I'd shoot several brands and styles to determine if your particular gun has a preference, concerning accuracy, for one or another. Any have enough energy.

Another consideration prior to buying the slugs......will you be using a smoothbore.....or a rifled barrel? And buy slugs appropriate for the barrel, (they are marked on the box in most cases)!

Good luck in your endeavor.

Dave

P.S. If it is at all possible try to select a gun with either rifle type sights.....or a scope. It makes accurate shooting sssoooooo much easier!