Calculations thanks to Wiki......
4 Bore (Black Powder): 1,750 grain projectile. Muzzle velocity around 1330 ft/sec. Equals out to 6,750 ft/lbs of muzzle energy.
.500 Nitro Express (modern smokeless powder): 570 grain projectile. Muzzle velocity = 2180 ft/sec. 5850 ft/lbs of muzzle energy.
Bet I could get a big piece of bar stock and machine it to fit those cartridges. Be legal too because it's black powder.

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--Sun Tzu