There are a couple of different types of bullet proof glass. Actually most of it is not pure glass. Polycarbonate is the plastic of choice for bullet proof applications. If you go into a bank you will often find polycarbonate separating the tellers from customers. Polycarbonate is nearly shatter proof. You can actually take the piece of plastic and beat on it with a hammer and it wont break.
Now a bullet has kinetic energy and it either pierces a material or imparts energy into the material through an impulse. So plastic that is 1-1/2" or about 38 mm thick will stop many types of bullets. There is also glass that is laminated with polycarbonate to give a material that is very resistant to bullets. The hardness of the glass with a material behind it that does not shatter provides a superior bullet resistant glass.
The bottom line is that most bullet resistant materials include plastic (sometimes acrylic is used but it is much more apt to shatter than polycarbonate) most of the time polycarbonate and when laminated it provides a very good bullet proof barrier.