There is some loading data for gauge rifles in Graeme Wright's second edition of "Shooting British Double Rifles", but it is unfortunately out of print. He's working on a third edition, I believe. Round balls and wads can be had from Track of the Wolf and black powder from the Maine Powder House. I'd start with shotgun level loads, say 3 drams or so, and work up to regulation or nosebleed, whichever occurs first.