Yes, but it was undoubtedly the german side that used gases and flamethrowers first and thereby made counterattacks with those weapons a legitimate tactic for the other site.
I'm not a fan of vilification of Germans in first world war, because all major sides wanted the war and were, as you said, not exactly gentlemen. But those weapons were first used by the Germans.
I believe that's the gun that the Americans called "the Trench Sweeper" because of how effective it was when you made it into the enemy trench during a raid.
It, along with anything high capacity and rapid firing, are perfectly designed for filling long narrow corridors with hot lead in a short amount of time.
Shotguns were just easier to carry. Still are, mostly.
To be fair, history is filled with people calling shotguns unfair for one thing or another. Like the punt guns that would take out entire flocks of waterfowl at a time...