It depends what you mean by "accuracy," mt. mike. As has already been mentioned, 6" - 8" or less @ 50 yd. is certainly achievable, but that also depends on user skill, given the likely absence of a rear sight and the fit of the RB to the bore. I.e., you'll need to determine the bore diameter before determining which RB diameter is best. E.g, my trade gun measures ~.615" at the muzzle and will shoot a .597"- (Tanner mould) or .600" RB (Lee Precision mould) accurately (using .014" or .018" thick patch). However, it didn't shoot a nominally .605" (.605" x .608") RB at all well. You may also find that a 1/8" lubed felt wad placed over the powder charge and under the patched RB improves accuracy a bit. Lastly, you may also wish to experiment with your powder charge, as some guns respond well to higher charges. I.e., without rifling, you essentially have a knuckle ball. A bit more speed may stabilize it.