Called the "Punt Gun," this firearm of unusual size (henceforth known as FOUS) could discharge over a pound of shot at a time, and dispatch upwards of fifty waterfowl in a single go.
The FOUS, while popular during the 19th and early 20th centuries, proved so good at downing ducks that most U.S. states had banned its use by the 1860s, for fear of depleting stocks of wild waterfowl. Which... I mean... obviously, right?
It bears mentioning that Punt Guns were never fired as depicted in the image up top, but were mounted on and discharged from small, flat-bottom boats called "punts" (hence the name). As you might imagine, the recoil on this shot-cannon was, well, considerable:
History is funny.
Cock of the hammer to Darren for pointing out this reddit thread, featuring the top image