The WWI German formally protested US 12 gauge trench guns as barbaric.
Yep. The story goes that the krauts complained to the Hague or whatever organization it was at the time, that Americans caught using the shotguns would be executed. American commanders laughed at them and said that any kraut captured with a flamethrower or serrated bayonet would be summarily shot. Story has it that the krauts dropped the complaint.