You guys are mostly correct. It is a punt gun, however it was used in the early days of football to try and "punt" a field goal. Honestly the game's never been quite the same since they disallowed it. But they did keep the punt in the game.

To defeat the Confederate Army?

To kill all the McCoys and/or Hatfields with one shot?

Punt gun used for commercial waterfowl hunting

The Michener novel Chesapeake has a few decent chapters which weave a narrative around the gun and its use. And its confiscation.

Used in WW I.

And for a quick follow up with a sidearm....

There are some fine punt guns on display at the Chesapeake Bay Maritime Museum in St. Michaels, Maryland.

it's name is Morse,
it gets used when the upstairs neighbor does her step aerobics after supper time.