Overall, bullet-proof glass is generally used to protect employees and objects of great value in banks, government buildings, convenience stores, liquor stores, jewelry stores, post offices, churches and schools. It has other uses, including protecting cars and transportation units.
According to Wisegeek.com, bullet-proof glass is a "type of glass that is built to stand up against being penetrated by most bullets." It is more precisely a bullet-resistant glass because it is impossible to build a consumer-level glass that can completely resist the impact of a bullet.
In the world of bullet-proof glass, two main types are used: laminated glass and polycarbonate thermoplastic. The laminated glass has been in existence since World War II, when it was used as armor on vehicles and to protect soldiers. It is a heavy and thick type of glass. Typically it is 4 inches thick, but can be made up to 8 inches thick. It is made by laminating thin sheets of glass together by using polyurethane or ethylene-vinyl as a bonding agent. The polycarbonate thermoplastic is layered between sheets of glass so that the force from bullets or an object will repel. If it were to be hit with a fist, hammer or ax, then those objects would ricochet off the polycarbonate thermoplastic. This type of bullet-proof glass is made under trade names such as Lexan, Armormax and Makroclear.
Armored vehicles have been reinforced to withstand combat and attack. They usually feature metal plates to protect the car and tires that will run even if they are flat. Armored vehicles also have bullet-proof glass.
A series of military projects are in the works that aim to create lighter weight, defensive forms of bullet-proof glass for their units. A new glass that is under way is the use of aluminum oxynitried on the outer layer (in place of the polymer layer), which enables a much lighter and more resistant glass that will protect military vehicles and aircraft.
One-way Bullet-proof Glass
Bullet-proof glass on armored vehicles has usually been one-way bullet proof glass, which means a bullet will be resistant on the defensive side but not the offensive side. The outside layer of the glass is a brittle layer while the inside layer is a flexible one. The bullet has to hit the brittle layer first and then the flexible layer will cause its energy to slow down immediately. But if the bullet is shot from the inside, hitting the flexible layer first and then easily piercing through the brittle layer, defenders can strike the offenders.
Transparent Armor Gun Shield
The transparent armor gun shield was built by BAE Systems, a British defense and aerospace company. It is a protective gun shield for soldiers who operate vehicle-mounted machine guns. The shield is used to protect the gunman and provide visibility while he shoots. It can be mounted on armored fighting vehicles.