There is a federal regulation that makes it illegal to hunt ducks or geese with a shotgun that holds more than three shells: 50 CFR 20.21(b). It’s also illegal to hunt them with a machine gun: 50 CFR 20.21(a), but then you’d have to be mad to hunt ducks or geese with a machine gun, right?
You’d also have to be mad to hunt school children or grocery shoppers or concert-goers with a machine gun. At least the ducks and geese have been given half a chance to get away. I guess the National Rifle Association didn’t oppose the three-shell restriction for hunting ducks or geese.