Tracer rounds from a machine gun are a spectacular site, and in April 1916 no-one would ever have seen such a thing before.

They would have been mentioned in eye witness reports had they been used.

Jim, the "artillery duel" nonsense was debunked in minute detail.
Helga's log reported no incoming fire.
The one pound shells fired over the buildings would land far beyond the Liffey.
One pound shells could not raise fountains of spray that "drenched the crew."
And much else.