A punt gun is an enormous weapon used for hunting flocking waterfowl. Actually, hunting isn't really the right word. It's used for harvesting waterfowl.
A punt gun is basically a shotgun, but it can be about 140 pounds and 11 feet long with a bore diameter of 1" to 1-3/4". So it's a very large shotgun. Or perhaps an anti-duck cannon would be the best way to put it. It's difficult for a single man to carry alone, and would be absolutely impossible for a human to fire from the shoulder. These are only the largest of the punt guns, they do come in more man-portable sizes. They're called punt guns because they are mounted on small, flat punt boats so they can be fired with something approximating accuracy.
The art of punt gunning was at its peak popularity in the first decade of the 20th century, but was eventually outlawed in the United States. It's still legal to practice in the United Kingdom and Ireland, though. The procedure begins with loading the massive shotgun onto a punt and casting off into a body of water where one is likely to encounter flocks of waterfowl. Once spotted, the punt must be very quietly and carefully rowed to within 60 yards of the swimming flock, careful not to surprise them or be noticed, which can be difficult in open water with little cover.
This requires a good deal of patience, strength, and endurance, as the punt gunner must keep as low as possible, lying prone in the boat while rowing as quietly as possible to stalk the waterfowl. Once within the gun's 60 yard effective range, the waiting game begins. The best time to shoot at waterfowl is when they are lifting, or just starting to take off, rather than floating on the water. Good reaction times are necessary, as well as accurate aim. Horizontal aiming is done by maneuvering the boat, since the punt gun is fixed to it. Vertical aiming is done by adjusting the elevation.
Once a good shot presents itself, the enormous shotgun sounds like a cannon going off. It can launch up to 28 ounces of shot (1-3/4 pounds, or 800 grams) accurately up to about 60 yards, with a spread of about six feet in diameter. The point being to kill every single duck in the flock with one massive volley of BBs. Aim is essential, there will be no second shot. If the shot was true, an entire flock's worth of ducks can be taken onto the boat, traditionally to sell on the market. The enormous gun can have so much recoil that the boat can be pushed backward a surprising distance.
This is in sharp contrast to standard shotgun duck hunting, in which the hunter must fire (ideally) one shot per duck from shore and collect them where they fall. A flock wiped out using a punt gun can be collected quickly and easily by rowing over to the carnage.
Obviously, it is far more dangerous to one's fellow hunters to fire bird artillery on a low, flat trajectory than it is to shoot a smaller shotgun into the air. Punt gunners must be extremely careful to check what is beyond their target and keep their eyes out while watching the flock they are stalking.
http://www.wildfowling.com/puntgun.htm
http://www.youtube.com/watch?v=iUiTBC0AfVE
smartalix tells me that the 1978 novel Chesapeake by James A. Michener describes the practice of commercial waterfowl harvesting with punt guns in the United States before the method was banned.