Hounds,
The trench guns are the marquee guns, but the shotgun manufacturers also supplied thousands of shotguns in all calibers.
Most went, believe it or not, to Army, Navy, and Marine Corps flight training schools, where fighter pilots and aerial gunners used them to learn the finer points of how to lead a target.
They'd start off on the ground, standard trap/skeet range, then they'd graduate to being the the bed of a moving truck with the thrower mounted on that truck, or another one.