Ok there is this occurance, but it still looks like the bullet went through, didnt the guy stick a screwdriver through it,
but even if it didnt there is no proof that this wasnt just a freak occurance, maybe the 20mm was fired very far away and lost a lot of velocity and just barely pierced the glass?
The 20mm bullet usually wasn't an AP round which is what would be needed to take out a gunner, although, 2 strikes near the same place should be enough to shatter the glass...
Think of it this way, shootin a BB at a glass from 10-15 feet away wont break the glass, it might crack it, or chip it, but wont break it.
But, if you put 1 BB in, and another about .5 in away from the first, that glass is gonna break, if only in that area.
I think this is a good idea.