Duckbill?
Hmmm, I thought it was called a trenchbroom.
Trench Broom was / does refer to the Mod 97 Shotgun (and maybe the Mod 12 as well) of WWI. I have heard the term with regards to the Thompson that was designed close too or was supposed to be used in WWI yet I don’t think it was used or issued until later after WWI etc .......
The duckbill I speak of is a device that is welded / brazed onto the end of a shotgun barrel to develop a shot pattern, looking at it from the business end it resembles a ....ducks bill.....:o)
I liked the Mod 12 as we could slam fire such vs the Rem 870’s we also had in the armory at the time. We had two Mod 12’s that had damaged barrels so my CO allowed me and the armorer to cut it below the dent at the end of the mag tube and add the duckbill. Good rig !!