An advantage to making your 500 JRH brass by cutting down 500 S&W brass is the size of the rim at the base. A 500 JRH has a turned down rim compared to the rim of a 500 S&W case. By using the 500 S&W cut down to 500 JRH you get a robust rim that will eject more reliably from a double-action cylinder. A double-action ejector can miss the rim of the factory 500 JRH and leave the empty casing resting in the cylinder. Not a good situation if needing to reload in a hurry...
He has a BFR in .500 JRH. Not an issue. I can say this with certainty, my .500 Linebaugh BFR is much harder to unload and load in a hurry than my .500 JRH BFRs because of the increase in rim size. It hangs up much more easily. I kinda like the smaller rim. Plus, only .005 separates the JRH from the S&W rim.
