If you've ever been on the receiving end of a big man with a gun, seen the bullet stop inches from your head with just a two inch web of splintered glass in between you and your still visible assailant, and decided to walk away without another thought to the technology behind your survival, well this node isn't for you.

Bullet resistant glass is an amazing feat of manufacturing. It is made by layering a polycarbonate material (like Lexan, Tuffak, or Cyrolon) in between pieces of normal glass. This lamination process results in a sandwiched material where the outside faces are brittle and the inside pliable. It is especially important that the index of refraction of the two phase compositions are nearly identical in order to maintain complete transparency. A bullet fired at this glass would break the outside glass layer, but the energy would be spread out over an area, allowing the polycarbonate layer to absorb the rest of the energy while holding the shattered point of impact together.

Bulletproof glass can be purchased ranging from 7 millimeters thick to 75 millimeters, and sometimes even more. The thickness required depends on what weapon you decide you are most likely threatened by, and your country of residence (in Middle Eastern countries where AK-47's are readily available, Level II protection does nothing for you). There are no established standards for testing armored materials, but the following is a generally followed chart of armor strength (fired from a 5 meter range):

LEVEL  TYPE OF WEAPON        CALIBER       APPROX. VELOCITY
I      .38 Special S&W       .38 Special   1050 f/s
IIA    9mm pistol            9mm           1175 f/s
II     .357 Mag S&W          .357 Magnum   1250 f/s
IIIA   .44 Magnum Colt       .44 Magnum    1350 f/s
III    AK47 Kalashnikov      7.62mm x 39   1350 f/s
IV     SLR L1A1              7.62mm x 51   2540 f/s
V      Dragunov SVD          7.62mm x 54   2750 f/s
VI     M1                    7.62mm x 63   1400 f/s
VII    5.56mm rifle          5.56mm        3080 f/s

One-way bulletproof glass is also available, where one side stops bullets, and the other side allows them through unaffected. This is made by laminating a sheet with one side brittle and the other side flexible polycarbonate. Bullets hitting the brittle glass will be stopped by the same mechanism in true bullet-resistant glass. Bullets coming from the polycarbonate side will pierce the flexible material at the point of impact, and continue through the brittle side with a small loss of velocity.

If you are looking to buy one-way glass or a protection level not mentioned in the above chart, please stay away from me and my family.

Props to http://www.howstuffworks.com/question476.htm

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