In Merry England, "bore" and "gauge" have the same meaning: The number (4-bore) refers to the number of round lead balls to a pound. Same for 12-gauge, etc. (Don't ask me how the .410 got in there.) These El Gigantico critters were mostly used in the early days of hunting in Africa.
(I'm no blackpowder expert, but I have read that in early America, a 40-caliber rifle was considered a small-game gun.)
Part of the Roy Acuff collection was on display in the Opryland Hotel in Nashville, last May. There were three or four four-bores there; one a double-rifle.
During the late 1800s, "market hunters" hunted ducks from small boats. Places like Chesapeake Bay. They would use long-barrelled "punt guns" of 8-guage and larger, shooting rafted ducks for sale to hotels...
As far as hunting African game with a .50 BMG, an important aspect is having a quick-handling rifle, light enough to be carried all day long across many miles of rough country. Another is surviving the recoil, and rapidly getting into action for a second shot. Do you know of any .50 BMG which fits the bill?
, Art