IIRC, I was some of these at the Winchester Museuem and they were used in the early 1900’s to bring down numerous geese from a flying flock with one shot.
They were mounted on their boats.
Those are slightly different. Those were called “punt guns” and they were smoothbore - a shotgun for killing a huge number of geese and ducks on the water’s surface. They were outlawed in many states in the east along with market hunting.
The 2-bore and 4-bores I’ve seen have rifled barrels, and they’re intended for use by absolutely daft Englishmen to go after elephants, rhinos, cape buff and the like.
Why anyone spends as much as these guns cost to have a new one today, to USE today... on a hunt in Africa today... is beyond me. But hey, if a guy is paying good money, it is incumbent on someone to fill the need if it is legal to do so.
I found pictures of a 1880’s 4-bore for you, with pictures of the rifling.
Warning: This link is VERY image-intensive, with many high-res photos.
http://www.hallowellco.com/rodda%204-bore.htm
This level of work is still done today, BTW.