For fun, I tried roundballs in my Parker Hale Enfield musketoon. It shot poorly. I'll stick with the minie balls casted from my Parker-Hale mould. Never tried those same minie-balls in my Springfield (I have an Armi-sport) though.
If you want advice from the experts on minie-guns, go to the North South Skirmish Association Website. Those guys shoot competitively with those guns.