I think really small balls have little use in hand guns because of the nature of the beast being primarily for self defense were as in a rifle I have heard of them down to .24 caliber working excellently on squirrel, rabbits and vermin to 25-50 yards max which is about all the distance such a small sphere could be of much use.
Muzzle blast magazine had a good article on small muzzle loading rifles last month.
I had previously heard of .30 caliber PRB shooters and had shot a .31 caliber a bit but never heard of any .24 and .25s before the Muzzle Blasts article. Mike D.