I have had good results using .535 PRB's in several T/C rifles. On the down side it is hard to start the patched ball. The rifles shot RB's very well with a tight fit. Even with marginal bores-neglected in the past-the rifles will shoot well. Early on I used a New Englander .54 successfully in local matches. I would suggest considering a GM drop in barrel for shooting PRB's. In is not necessary to fight the ball down the bore with GM's.