“Bulletproof” glass is very different to normal glass
More correctly called bullet-resistant glass (because no glass is totally bulletproof), it’s made from layers of tough plastic called polycarbonate sandwiched in between pieces of toughened glass.
This sandwich of layers is called a laminate. It can be up to ten times thicker than a single pane of ordinary glass and it’s usually very heavy. When a bullet strikes bullet resistant glass, its energy spreads out sideways through the layers. Because the energy is divided between a number of different pieces of glass and polycarbonate interlayers, and spread over a large area, it is quickly absorbed.
The bullet slows down so much that it no longer has enough energy to pierce through – or to do much damage if it does so. Although the glass panes do break, the polycarbonate layers stop them flying apart. Think of bullet resistant glass as “energy-absorbing” glass and you’ll have a good idea how it works.
This is the same principle for the framework. The heavy duty steel sections are heavily reinforced with sandwiches of steel, therefore when a bullet strikes the steel, its energy is quickly absorbed.