500JRH is 500 bullet diameter, and proper headstamp ammunition is available from Buffalo Bore. You can also cut 500 S&W brass to work with the JRH

454 Casull is a hotrod, nothing wrong with it for a powerful wheelgun.

For the single action revolver, with the shorter barrel length at a point come problems with ejector rod length.

I could show you some details of a 3-1/2" 480 build that address that issue if you like.