Despite several years of study and effort - and quite a bit of encouragement from people who claim true three or four MOA from the things - I've never gotten consistently good results.
Well, those claims might be just that. I have heard them claims. I do know from my own efforts that more velocity will keep the ball on course longer, but there's a limit. My Brown Bess is certainly more accurate with a 140 grain charge, compared to 100 grains or less. Realistically, I don't think one is going to keep a smoothbore on a paper plate much past 70 yards. For humane kills on large game, I don't think shots over 60 yards are very ethical.
Playing around with my paint ball gun one day, on the balcony of a house that was on a steep hillside, I could watch the flight of the balls real well for a long distance. What they do, is fly real straight for a certain distance. Then you see them veer off, randomly, suddenly right or left, high or low. Not a constant trajectory, real straight, then, hang a left! Or right. I believe a lead ball behaves the same way. With a musket, one can get really good, excellent accuracy out to 50 yards, or a little more, but go to 100 yards and they spread out, far more than with a rifle, with "cone shaped" dispersion. So, I'm not sure what kind of magic or voodoo can keep a non-spinning ball in a three or four minute of angle. ? Mike Belivue (SP?) has quite a few videos of shooting different muskets and smoothbores out to 100 yards with different loading techniques, powder charges, etc. I think they are quite representative of actual, realistic smoothbore accuracy. You can certainly hit a man at 100 yards, 95% of the time, where any hit is a good hit, but one is not going to place the ball reliably into the vital area of a deer, or keep your shots on a paper plate. Very likely to blow off a leg, hit it in the butt, etc.