Do we even want to know how you know that?
At Ft Benning GA we have an artillery adjustment range- students call for and adjust artillery fire from a battery of 105mm howitzers. Live fire. Sometimes the students screw up, and the shells hit their own position.
So we built them a bunker. It is a LOT of very tough steel and fiber reinforced concrete. The ground level windows in the bunker are Lexan. 12 layers of 1 inch lexan. Now and again, we would need to replace the outer layer.