A few years back there were two or three custom 'smiths who offered cartridge changes in the 50 express. I recall seeing one very nice conversion on a Browning 1886 with pistol grip and 26" barrel shortened to 22". I can't recall if the barrel was roll marked or inscribed with the new cartridge moniker, but it was a tip off that what this was was a custom gun.
I recall one load was the 450grain Barnes flat point at an honest 1750 fps. I just can't recall who made the hollowpoint for the 300 grain express load, but this was good for 2,200 fps with smokeless powder, of course for both loads.
To say that this gun thumped your shoulder was an understatement. I imagine you could use this gun on anything that walks-or runs. What do they want for it?