Good article in this month's American Rifleman about trench shotguns. Back in WWI, everyone was looking for more firepower for the infantry. The Europeans played around with a variety of unreliable sub machine guns. The Americans went for good old 12 gauge pump action shotguns. I saw one of these babies for sale at InterarmCo in Alexandria VA many years ago. It was worn, but the bore looked humunguous, and it had US Govt markings on the action. Not something one would care to face in a trench. Or anywhere else for that matter.
Machine guns achieve rapid fire thru a complex action that has to cycle without fail for each shot. Shotguns achieve rapid fire thru a much simpler design, they simply pack a bunch of lead balls into one big barrel. Notice that shotguns are still for sale at Walmart, where as the Tommy guns, the Schmeissers, the Stens, the grease guns, and their ilk are now museum pieces.