Go to the Hodgdon Reloading Center on the web. Hodgdon Longshot powder is probably going to be your best choice.

One my favorites from their recipes:

12ga 2 3/4" Win AA case:

Bismuth 1 1/4 oz. 26.4 gr Longshot powder, Win. 209 primer Rem. RP12 Wad, 7,500 PSI 1250fps