I'd think a slighty lighter ball might shoot better than a full, bore sized ball due tothe convergence of the tubes. An undersized ball is what shot well in my own 12 bore double.

For that, I'd have Jeff Tanner make me a .750" ball mould and I'd use a system of wads, with the base off a plastic steel shot wad for the cup to center the ball in the bore. The wad will 'take' the spin and impart it to the ball, with marking the ball less than 1/2 rifling depth - good scenario with no or minimal leading. With 40 points of choke, a standard 10 bore should have a .734" bore. That's only .008" engraving each side for the ball, instead of 20 thousandths per side for a bore sized ball (.774") through a full choke.

Start with lighter, fast shot loads and go from there. I'd think Blue Dot or perhaps H4227 might be good powders. I'd go with the hottest shotshell primer you can to help with ignition, due to the lighter weight of the ball, over normal shot loads - I think it's called a #439 Federal. The Black Powder and Pyrodex shotshell primers are not hot, I've been told, but are actually weaker than standard 209's. I've no experience with them.