The local history museum in Havant, just up the road from me, has this formidable 9ft long duck gun propped up against a wall.

It was made some time in the 1890s by James Pycroft, a member of a large family of farmers, builders and sailors on Hayling Island. James himself worked as a fitter in the naval dockyard at Portsmouth, so he had access to the machinery to make duck guns properly - a great many wildfowlers must have been killed by their weapons over the years. This one even had a breech loading mechanism, which the museum is trying to locate. James built this particular gun for his brother Harold (pictured), who built several punts of larch, sitka spruce, deal and parana pine. According to Harold's son Noel, the gun would be loaded with 2lb of BB shot for Brent geese or No1 shot for widgeon, duck and teal. Blasting powder provided the propellant.
Harold and his brother Albert shot 3,000 Brent geese over the bitterly cold winter of 1939/40, which were sent to a game merchant in London. Noel recalls being sent out onto the mud looking for barleycorns mixed up in the bird poo - apparently, seabirds cannot digest barley so this would be an excellent indication of where they were roosting. A dawn raid would then be mounted.
This picture is interesting for two reasons. One is that the punts seem to be in more ice than I have ever seen in the harbour. The other is that they are fitted with outriggers, presumably to make rowing to the killing ground a bit faster. The final approach would have been made by running one of the oars back over the transom and sculling.
Many thanks to the curator at Havant Museum for pointing me to the right records, and for permission to take the pictures.