I am not convinced that the gun is restricted to elongate bullets. I have what I believe is a Lancaster rifle which shoots a .60 x.66" belted roundball (patched). I usually use 70 grains of 3F powder with a 20 guage cushion shotgun cushion wad between powder and ball. I may have measured the twist rate but I don't recall what it is. The gun is accurate on trail walks so I don't worry about the twist.
It is not difficult to add a belt to a nested mold such as Jeff Tanner makes.

cheers Doug