In a smooth bore, you want a ball diameter than is at least .020" SMALLER than your bore diameter. Some find going down to .030" and using a thicker patch material gives a better working load. The balls will seat easier in the muzzle, and all that lube in the thicker patch material helps to keep the bore clean. The big caliber smooth bores, like yours, offer a lot of lead to UPSET and expand, quickly, and enough to fill these large bores to provide the needed gas seals. If not sealed well enough, use an OP wad over the powder to do the sealing, and use that undersized ball and thick patch to make loading easier. You are basically making a gun to shoot a RB accurately enough to kill a deer out to 60-75 yards, Tops. Using a tighter ball and patch combination might tighten groups up a bit, but with that large, heavy ball, the deer is Not going to know the difference. "Accuracy" And Smooth bores are terms not normally associated within the same sentence. If you can get a load that will group 5 shots in a 3 inch circle at 50 yds, you are ready to go hunting.
And, stay away from the massive powder charges, and the fast burning powders. They aren't needed. An ACCURATELY PLACED .58 cal. RB will kill the deer just as well as a faster ball, that may not be so accurately placed. In that caliber, its the huge weight of the ball, and the caliber sized hole that does all the "killin'". At the short range these guns are accurate, you don't need lots of velocity. An 80 grain load of 2Fg powder is going to kill a deer just as much as 100 grains of 3Fg, but it won't hurt your shoulder. :hmm: :thumbsup: