A punt gun is a type of extremely large shotgun used in the 19th and early 20th centuries for shooting large numbers of waterfowl for commercial harvesting operations. These weapons are characteristically too large for an individual to fire from the shoulder or often carry alone, but unlike artillery pieces, punt guns are able to be aimed and fired by a single person from a mount. In this case, the mount is typically a small watercraft (e.g. a punt).

Source: https://en.wikipedia.org/wiki/Punt_gun

A more common spelling for the name of the activity is "wildfowling." But I can't find the damn song. Here's a video of wildfowling with a punt gun:

https://www.youtube.com/watch?v=nOkrprr2kwg (warning: dead birds)