Letter Re: Lexan for Bulletproof Windows

Dear James,
I have recently purchased raw land to build my retreat. Soon I will begin building a home, and wish to equip it with windows which can resist small arms fire. I can obtain Lexan in 1/2″ thickness, and my question is, will I need two pieces of glazing in each window, or three (or more)? I do not think it likely that I will be shot at with anything larger that .50 caliber. Your thoughts on the matter are most welcome. Thanks, – Zoomer

JWR Replies: To begin, I must warn readers that acrylic Plexiglas and polycarbonate Lexan are significantly different materials. Lexan is flexible, while Plexiglas is quite brittle. Some other flexible transparent polycarbonate plastics include Armormax, Cyrolon, Hygard, Lexgard, Makroclear, Makrolon, and Tuffak. So only use one of these for ballistic protection applications, not Plexiglas!

Your intent to use multiple laminations of 1/2″ thick Lexan is not without precedent. But its sounds easier than it really is, in practical application. One sheet (of 1/2″ thickness) Lexan will stop single hits from a .22 Long Rifle (LR) rimfire, but not repeated hits if they are well-aimed. Two thicknesses will stop 9mm, but they won’t stop any bullets at higher velocity. Unfortunately, it would take more than 3″ of just Lexan to stop most rifle bullets, and probably much more than that to stop .30 caliber steel-cored AP bullets from a 7.62mm NATO, .30-06, or 7.62x54r. And I would assume that stopping .50 BMG AP or API would require more than foot of thickness of just Lexan, but I haven’t been able to find an unclassified source on this. For comparison: the Springfield .30-06 produces a muzzle energies up to 3,000 foot-pounds, while .50 BMG ball produces up to 15,000 foot-pounds! (An unclassified industry white paper Sierracin/Sylmar Corporation is quite instructive. Detailed ballistic protection specifications for military armored glass developed by the US and UK military are classified.)

The armored glass used in many current lightly-armored vehicles such as the up-armored M1114 HMMWV are up to 3.5″ thick (depending on armoring generation), and use proprietary sandwiches of transparent polycarbonate plastics and laminated glass. Lighter-weight armored glass made for limousines are even more exotic (and costly), but are still quite thick and heavy.) One of the very best is Global Security Glazing’s Secur-Tem + Poly, which has been tested to NIJ Level IV protection against single .30-06 hits. But even this is still 2.11 inches thick, and it weighs 24.38 pounds per square foot. The cost per square foot for this material is quite high.

The most efficient bullet resistant windows are made by bonding alternating layers of Lexan and laminated glass. Note that if you are making your own, that the inner-most layer should always be Lexan rather than glass, to prevent glass fragment spalling. (Just because a bullet is stopped, doesn’t protect you from getting splattered with fragments, as the inner-most layer flexes with a hypersonic shock wave.) It is notable that most modern armored vehicles have a spall liner.

So, say that you want to build a house with “.50 caliber bullet proof windows”? Unfortunately, the cost of even .30 AP protection would probably be prohibitive for constructing any residential windows larger that 12″ x 12″, and even then their transparency would definitely suffer. With more and more laminations, a window becomes progressively more opaque–that is, translucent rather than transparent.

Lastly, you need to consider that the window frame that you use will have to be wide, very stout, and very firmly attached. Otherwise, your window laminate will pop out with the impact of the first shot, leaving the opening unprotected. Windows with narrow or otherwise unsubstantial frames would also be vulnerable to attack by sledgehammers. A wedge shaped cross-section (achieved by making the outer layers progressively larger surface area sheets, and a tapered window frame, to match) is the most effective way to protect against such attacks.

Reader George S. wrote to add: The Schott Glass/ GEMTRON Vincennes, Inidiana glass production line has been shut down, but their production of laminate glass direct vision panels for the recent-generation Oshkosh military MRAP armored vehicles is still in operation.