You could always do what I did. I have a percussion Great Plains with a 54 GPR barrel and a 50 GPH barrel. That way I can shoot the larger PRB if I want or shoot conicals or sabots in the 50. I just got the GPH barrel and hope to shoot it in the next week or so and start working up a load for it. I got the extra barrel from Midsouth Shooters Supply for under $130. They also have good prices on the whole gun. D&R is another good source for the gun.

Good luck with what ever you decide.