About 20 years ago there was an article in "Shooting Times" about a punt gunner living on the Severn Estuary who built his own punt gun out of stainless steel tubes.
I can't remember if it was 2 or 3 barreled but I do remember that it produced a wide side to side pattern to deal with rafting ducks. Perfectly legal in England at that time. Not sure if that's still the case. It's effect on the bird population would have to be minimal. Not to many nutters want to do it anymore, Spam is cheaper.