Those are punt guns .. used by market hunters for waterfowl. They're still legal in England.*
Chesapeake Bay was one of the places they were used when canvasbacks were bringing $1.50/brace.
The method used by the hunters, called marketmen, was to go out on moonless nights and shine the rafts of ducks with a candle reflector. The ducks were confused by the bright light and they would touch off the punt gun and kill dozens with one shot.
A larger swivel gun, a small cannon, was sometimes used.
* "Punt gun use continued into the 21st century in the UK, though laws limited bore size to 1.75″ or 44 mm, as opposed to the more common 2″-50 mm. Ever since Queen Victoria’s 1897 Diamond Jubilee, there has been a celebratory punt gun salute over every English coronation and jubilee. Several sporting manufacturers in the UK commercially produced small runs of punt guns, which were of better overall quality than those custom (read, homemade) jobs in the US. Though ours were often crudely built and hard on the eyes, the guns had to be sturdy and made to withstand rigorous hunting use."