The Great Plains Hunter is a 1-32 twist and is set up to shoot conicals and sabots. Start your loads at 80 grains and a .54 caliber maxiball and see what happens. Some other loads you might want to try are Great Plains Conicals, Powerbelts, and some No Excuse Conicals. The rifle should shoot one of them well. Also if they are flat based you might put a wonder wad under them.

You better keep that wife, when they buy guns like that they are few and far between...