A punt gun is a huge shotgun for hunting large numbers of waterfowl. Though popular in the 19th and 20th centuries, the use of a punt gun for hunting is now considered "market hunting". Because it too quickly depletes stocks of wild waterfowl, it's largely forbidden.
Punt gun bores could exceed 2 inches, and they could fire over a pound of shot at a time. They were called punt guns because, too big to fire held in hand, they were mounted to small hunting skiffs (called "punts"). Hunters using punt guns would often work in groups, quietly maneuvering their boats in range of a flock of birds, and then all firing their punt guns at once.
Nowadays, punt guns are still used in ceremonies or owned by collectors.
In addition, the punt gun is a good choice when performing the age-old tradition of cleaning your gun when your daughter brings her new boyfriend over.