So it seems like the US began using pump action shotguns to great success in its colonial wars and so had no qualms about bringing them to Europe.

These trench guns were extremely useful and the German government lodged an official complaint against the US about them being excessively cruel weapons. The US ignored this complaint because ... poison gas warfare.

Are 00buckshot wounds noticeably different from shrapnel?

Was Germany legitimately concerned or was this a diplomatic maneuver to buy time or negotiate or anything more to it?

Did Germany test their own trench shotguns for stormtroopers to fight fire with fire?