What Matador and GMB54 says.
I like to call the Huntsman the first "modern muzzleloader". It was pretty much designed to be a round ball gun as its twist was somewhere around 6o" or so. Those sabots mentioned are cheap enough to try but I have my doubts about you being able to stabilize a sub caliber bullet with that slow twist. Bore size conicals may give you suitable accuracy. I think you can get Buffalo Bullets or even maxie balls from Dixie Gun Works or Track of the Wolf or you may even try Cabelas. If you were closer I'd give you some minie balls to try as I been known to shoot a Civil War era musket in .58 a time or two and have a few laying around.
One other thing about the old Huntsman that you may or may not be aware of but would be good to know. It has a rather unique breech plug that is not threaded but pressed in and held in place by an o-ring. As long as the action is closed, when it fires, all is fine. However, if the action were to be opened, say immediately after a hang fire, and then it subsequently discharges, the breech plug would be violently ejected in the general direction of the shooter...not good.