Well, the Musketoon was not designed to a a smooth-bore, so the item that
you have is a compromise-style of a shotgun, as the bore has simply been opened out by removing the rifling. You should aim to use a ball of the diameter that uses a pillow ticking patch of around 0.015", suitably lubes either with spit, or a proprietary lube [spit is cheap, and you can take it with you].
Do NOT attempt to shoot any kind of bullet-shaped projectile in this piece - not only are you wasting your time, but also expensive powder too, as the results will look like a fire-hose. A properly-patched and good-fitting ball will be plenty accurate at up to 50 yards or so, but it sure as heck is not going to be nice to shoot.
Reading THIS thread might offer you some help -
http://www.airgunbbs.com/showthread....Hale-Musketoon, but that ore of yours sure is oversized, even for a smoothbore!
tac
PS - What do you mean by 'solid heads'? What are you shooting now that has a separate head to the bullet?
PPS - The accepted terminology is RPB - round patched ball.