As to wads, Martin- the AA is about the best as to shape of the base, gas check. Some guys, Lyman, for instance, use the wad as-is, with the petals cut off, but this system leaves no method for keeping the ball centred. You would possibly cut the petals off, then put a deatched (petals and figers removed) base cup under the ball, with the ball sitting in the cradle of the gas check. Height is the only problem with this, but might work perfectly, as the ball has less height than the shot column. The 'extra' base wad (about 1/4"high) sitting on top of the AA wad's colum might give the correct height afterall. To get a proper crimp, one needs the correct height - experimentation is generally needed. I find if the wad column is slightly high, or slightly low, is OK. I run the case back into the crimper station to get a nice outside radius on the case mouth. This is less needed on a non-repeating 12 bore, ie: a single, over/under or side by side.

Now, a 3/4" wad punch, easily made from a 3/4" or somewhat undersized 13/16" Inside Diameter pipe, possibly case hardened will allow you to cut your own filler wads from card board or donnaconna house covering.

A .715" ball will be OK and will cast about 545gr. in pure lead and 515gr. in WW metal.

If you can get SR4756, Hodgdon's Longshot, Blue Bot or Herco, you should be able to develope smokeless loads running 1,400fps to possibly 1,600 fps with that ball. These will duplicate the 7 dram HEAVY load for African BP 12 bores mentioned by Greener. I'd have no qualms of taking on one of your buffalo with one, using a straight .715" WW or hardened WW ball.

There was one heavier load for the 12's, a 9 dram load, which would have developed perhaps 1,650fps, maybe 1,700fps but probably the lesser of the two.

BTW - shooting even a 7 dram load of BP in a light 12 bore is memorable - and hard on the stock wood. A velocity duplication load in a modern 12 bore gun, easily accomplished using smokeless powder, kicks only about 1/2 as much as a black powder load.