Seems many guys with these big guns shoot factory ammo. Those who shoot mainly or only handloads can chamber to anything they want and if the brass isn't made, make it.

The Rugers are easy, just go with a shortened .45 3 1/4" case and a .45/70 extractor - or stay with full length brass and make 2,400fps or more with 500's. Turn it into a gopher gun with Barnes 250gr. "X"'s.