Yes thanks. You should have received a reply. I will be looking out for those WW1 guns and taking lots of photos. I am going there on Wednesday afternoon.
From my limited reading about Tankgewehr rifles they were introduced from May 1918. I would have thought the most likely targets would have been Mark V tanks
which had thicker frontal armour (16mm vs 12mm). I thought the video test was pretty contrived - getting within 50 yards with a crew of two and a 2m rifle during a tank attack doesn't seem realistic to me.
I suspect that it was more a matter of the tank getting within 50 yards of the Tankgewehr. I think the idea was for the Tankgewehr to give the infantry a weapon with which they could defend themselves. As such it had to be portable enough for a German unit to move it quickly from one part of their trench to another as the tank approached. This could be done within the cover of their trench.
I think medals would have been in order for anyone dragging one of these things out of the trench into no man's land, hunting down a moving a tank.