How would this get infantry across no. Man's land?
It's what it would do after the infantry got into the opposing trenches.
Which was the easy part.
Massed infantry charges routinely got into trenches. Just kicked out through slower reinforcement
After the US got into WW I in 1917, "trench guns", usually Winchester model 97s and model 12s were issued to troops. Loaded with #4 buck, the load still used today by the military. A 2 3/4 inch shell with 21-23 .24 (6mm) pellets. The Germans complained about the use of such weapons but the US response was, "Send more shotguns". They were a damned effective weapon in CQC even though they were pump actions and slow to reload. A magazine fed full auto 12 gauge would have presented a significant advantage in trench battles.
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