100% sure that guns is a .500 yes, the only question is case length. You can see from the pictures it is a straight walled case so that takes any of the 577/500 variants out of the equation. So now we need to figure case length, the best way being a chamber cast on a gun from this vintage but I am betting on 3" as it was by far the most common, though it was made from a 1&1/2" case all the way up to a 3&1/4" case. On page 220 of the 3rd edition of Graeme Wright's book "Shooting the British Double Rifle" you will see his regulation load for the .500 3" of 136 grains of bp and a 340 grain bullet, this is a standard load. These same loads are confirmed in " British Sporting cartidges A summary of case types, headstamps, bullets and charge variations" by Bill Flemming.
I am 100% comfortable saying that you will not find a vintage British black powder double in .500" any length cartridge that was regulated for a 715grain bullet with 68grs of powder.
I'm not a gambler, but on this I would bet the farm.
Steve
