12 bore rifles:
Getting useable plains game/Forsythe velocity with a ball this big is going to make a lot of recoil in a hunting weight rifle. Forsythe's rifle used a "#15 ball".
12 to the pound ball weighs 580 gr and is .730 in diameter so. A .720 ball will weigh 560 grains this is 30%+- heavier than a .662 ball (16 to the pound). It will require more powder, probably 30% more to make the same velocity a 16 bore will.
I have a 80" twist flintlock rifle that uses a .662 ball and a heavy patch. It makes about 1600 with 140 gr of FF Swiss.
It weighs about 10 pounds and makes all the recoil I care to have. It shoots flat enough to about 120 yards.
Its also nice to have a bore size that will take a ball that is readily available. I have a tanner in .673 but its a bigger pain to use than a Lyman mould.
Dan