Explanation: After some three years of European powers brutally bludgeoning, bayoneting, machine-gunning, and using chemical weapons on each other in WW1, the Americans entered the war. We, of course, immediately indulged in the same bludgeoning, bayoneting, machine-gunning, and chemical weapons as every else did, but we also brought a large quantity of pump-action shotguns to the fight, with brutal effect in the tight confines of the trenches.
The German Empire lodged a formal complaint that the use of shotguns was against the laws of war due to the inhumane nature of the weapon. "Lol", said everyone else, "lmao"
To be fair, weren't shotguns primarily intended to be used as hunting weapons?
Yep! In fact, the Germans had a long tradition of making fantastic single-shot, and double and triple ( 'drilling' ) barrel shotguns for hunting, even using them during WW1 - to hunt behind the front lines and supplement the food supply.
But it was the (relatively) lightweight and fast-firing tube-fed pump-action shotguns that the Americans brought over in large numbers that were truly useful in a military context.