Round ball is a way to go, I have a 12 sxs and was given a 6 cavity 12 round ball mold , the balls passed nicely through the cylinder choked barrels, 480gn so about 1 3/16 oz. I figured that if I used a 1 1/4 oz powder reload charge the round ball would give a lower pressure than the accepted 1 1/4 oz recommendation being 1/16 oz lighter. I loaded the powder then used 1 1/18 plastic wads with the petals cut off just below the midpoint (widest) point of the ball. The wad as such stabilized the ball fairly central to the bore. My Chronograph was on loan to a buddy at the time but the old double put 2 balls from each barrel - four total, into an area the size of a cigarette packet at 50 yards say 2 3/4 inch by 3 inches. The ballistic tables for the load suggested about 1250 fps for the 1 1/4 oz shot load but may have been slightly higher given the slightly lighter shot / ball weight. burn was clean and no signs of pressure on the fired cases.
The loaded rounds were all roll crimped with a traditional hand cranked crimper, no over ball wad, the plastic wad compressed while crimping then pushed the ball back up against the crimp when completed, giving a very even crimp pressure across all the loaded shells.