Shoulder fired vs mounted in a "Punt" is probably a good one.
Though you could have a swivel mounted gun.

In this month's "The Field" magazine, a fellow was shooting 2oz of BB shot from an 8 bore single.

I'm not an expert, people on the eastern shore are more knowledgeable about them than a lowly Saginaw Bay waterfowler, but there is a practical charge limit for shoulder fired shotguns.
Once you go above that, it has to be mounted to the boat.

Out there doing it best I can.