I believe the longest cartridges in a revolver with a barrel length under six inches are unable to effectively burn the powder and achieve great velocities. I own a S&W 460V with a five inch muzzle brake equipped barrel. Shooting 454 Casull it probably comes out close to equal when using full out 460 S&W magnum in that short five inch barrel. But it is able to shoot 460 S&W magnum so I do.
TII Armory markets the 500 Bushwacker and is a lengthened 500 S&W magnum. They are out of Australia and TII Armory has a website that you can contact them to get your BFR altered to hold the 500 Bushwacker.
I saw a You-tube from TII Armory and it was them showing the cartridge and comparing it to the 500 S&W magnum. Looks to add about 600 fps or more of 500 S&W magnum using the same bullet weights. Google TII Armory and see for yourself.
This company can also rechamber your 500 S&W to 500 Bushwaker...I think it holds roughly 18 more grains of powder than the S&W....it's a true .500, unlike the 50 Alaskan, and uses handgun powder, unlike the 50 Alaskan.
The 500 Bushwacker can only be built on the BFR Long Cylinder frame and it is easy to do on a 500 S&W long cylinder BFR. But TII Armory can make any BFR long cylinder model into the 500 Bushwacker. The muzzle brake they attach to the barrel is mandatory. They say it cannot be shot safely without the muzzle brake. The Double action X-frame revolver from S&W cannot be made to work with the 500 Bushwacker cartridge.