Lubing Roundballs

You do not need to lube round balls. Round balls are almost always fired with a patch from muzzleloading firearms. The patch is lubed so the ball does not need to be. If the ball is being fired from a black powder revolver, you do not patch the ball. You should be sealing each cylinder chamber with some sort of grease or lard or a commercial product like bore butter to guard against accidental ignition of adjacent cylinder chambers. Some of this lubricant is blown ahead of the ball to lube the barrel. In the end a ball presents the absolute minimum bearing surface on the inside of the bore of any shape projectile. Even if you did lube it there wouldn't be much lube available on the bearing surface.