I have been playing with a 60 caliber round ball rifle and up until I ordered some larger balls from Track of the Wolf, have been shooting .57 R\B's that I had on hand.
My accuracy with the .57's has been on paper out to 40 yards but I was all over the place until I started double patching .20 thick patches. Still not real thrilled with my results!
All Track had for a 60 caliber was either a .58 or a .595 ball in their inventory, so I just ordered a batch of each to try. Once I figure out what my new rifle shoots best with, I will order a mold.
Trouble with these odd calibers is that you dare not do any sight adjustments until you have the proper shooting materials on hand to experiment with.