I'm not even sure what a proof table is or how one is relevant to getting a vintage British bp rifle up and shooting. I'm saying that gun was built for a variant of a British .500" cartridge, almost certainly the .500 3" and that you will never find a vintage British .500" BP rifle regulated for the load you quoted. These are known loads which can be seen in several loading and cartridge reference books. A 715 gr bullet is far heavier than British bp .500s were loaded. To get that gun to shoot, slug the bores and cast the chambers then work up a bp load that will regulate. I would start with a bullet of about 340 grains myself.
Steve
