I'm running two "20's", a NW Indian Trade Musket smoothbore and a 1740's .62 cal German Stutzen rifle. Both, of course, flint. I've taken one deer with the NW gun and a number with the Stutzen. In every case, the ball goes in one side and out the other, smashing everything in its path. Both are using a .610 round ball on top of 120 grains of fffg. With its wide butt, the Stutzen is a joy to shoot. With its flat, narrow buttplate, the NW gun seems to hurt you damm near as much as the deer!

.61 round balls will do everything you need.

Tom