.600 or .610....(or even .590 if you're using a stiff paper cartridge tube. )
Some folks swear by a patched round ball in a smoothbore.
I don't for a couple reasons. First, there aren't any known accounts of folks using a patched round ball in a smoothbore. Could it have been done? Sure. Was it done? Maybe. ...,
But I suspect that it wasn't done, and that's why it's not been documented..., and I think the
reason it wasn't done was because the patch in a rifle is helped to remain symmetric by the grooves in the barrel, which allow for
a uniform folding of the cloth patch. You don't get that uniformity in a smoothbore, and I think that impacts accuracy.
Now you can make a paper cartridge that will act like a shot cup and have a uniform positioning of the ball, or you can do other things to have a more uniform positioning of the ball, rather than to use a patch. I've seen folks dimple a ball with a rasp, and I've also seen folks modify 1/2" wads to have a concave portion in the center, that seems to center the ball and hold it.
The
secret to the dimpled ball, I've been told, is not to simply dimple it and think "it's like a golf ball now", but to actually raise tiny bits of lead on the ball surface, so that when you load the ball, these tiny bits of lead have to be swaged a small bit for the ball to fit..., which then means the ball is uniformly placed into the bore, and should fly more consistently.
LD