In light of recent events the use (or lack thereof) of “bulletproof” glass in schools has come under fire. Let’s start by addressing the fact is that there is no such thing as bulletproof glass. What movies, the media, and pop culture commonly refer to as bulletproof glass is in fact bullet-resistant glass.
Bulletproof would indicate that no matter how many, and what kind of rounds are shot at the glass, it would remain intact. This is practically an impossible task. Given a large enough caliber gun, or enough round of ammunition, any glass will succumb to this violent force. This becomes especially true when you take into account the variety of rounds that are available. Explosive projectiles would destroy some “bulletproof” glass within a split second. The same concept holds true for hurricane resistant glass as well- given a strong enough storm with high winds and flying debris, no glass could withstand prolonged damage.
Hurricane resistant glass, which is closely related to bullet resistant glass actually played a crucial factor in the recent school shooting in Parkland, FL. The shooter attempted to create a “snipers nest” but was unable to do so because after firing 16 rounds at a hurricane resistant window, it still did not break out, therefore thwarting his efforts at creating a vantage point to shoot from.
Let’s start with a history lesson. Bullet resistant glass was actually first used during World War II in bunkers and on armored vehicles. But the process was crude- it involved layering numerous panels of tempered glass together with epoxy to create panels up to 4 inches thick. At this point, clarity was compromised and volume and weight made use prohibitive.
After World War II, bullet resistant glass evolved because people realized that it had applications beyond the battlefield such as in banks and other secured areas. Although acrylic (more on this below) had been around since the late 1800s, advancements in the manufacturing process turned it from a brittle plastic into a strong material which was, at the same time, relatively easy to shape and work with.
So how is modern-day bullet resistant glass created? There are 3 types of materials that can possibly make up these panes.
Acrylic is a type of extremely hard, clear plastic. Acrylic achieves bullet resistance through sheer thickness. Most bullet resistant polycarbonate is at least one inch thick and because of its density, is quite heavy. Because of its hardness, acrylic’s defense is to deflect any projectile’s energy and cause it to bounce off.
Polycarbonate is also a type of plastic, but much softer and lighter- approximately 1/3 the weight of acrylic. Because it is a softer material, polycarbonate’s purpose is to absorb energy from projectiles and slow them down enough so that they do not pass through the material completely. Polycarbonate is layered, and many times the projectiles will become lodged within the layers. It is used in conjunction with glass to create glass clad polycarbonate which is also known as laminated glass.
This is often what people think of when they think of “bulletproof” glass like what’s seen in movies. When impacted by a projectile, there will be an obvious point of impact where the outer layer of glass splinters. But hopefully the inner layers absorb the energy, stop the projectile, and don’t let it pass through the other side.
Laminated glass is basically a glass sandwich where there are 2 pieces of glass on either side with a soft layer of polycarbonate layered in the middle. You see this almost every day whether you know it or not- this is the same way that the windshield in your vehicle is made. However, for bullet resistant purposes, layer upon layer of polycarbonate and glass is used to created the energy stopping power.
With all the material types above, thickness is what matters most. The thicker the bullet-resistant glass, the further the projectile must travel, and the more energy will be lost.
Underwriters Laboratory (UL) has created standard security levels based on their testing. UL is the nation’s top third-party consumer safety testing facility and test thousands of common household products. UL created UL752, the Standard of Safety for Bullet-Resistant Equipment. The chart below lists the levels and the forces they are able to withstand in order to pass UL’s testing.
|Level||Ammunition Tested||Ammunition Mass||Ammunition Weight in Grams||Minimum Velocity (Feet per Second)||Maximum Velocity (Feet per Second)||Meters per Second||Shots Withstood|
|1||9mm Full Metal Copper Jacket with Lead Core||124 grains||8.00||1,175||1,293||358||3|
|2||.357 Magnum Jacketed Lead Soft Point||158 grains||10.20||1,250||1,375||385||3|
|3||.44 Magnum Lead Semi-Wadcutter Gas Checked||240 grains||15.60||1,350||1,485||411||3|
|4||.30 Caliber Rifle Lead Core Soft Point (.30-06 Caliber)||180 grains||11.70||2,540||2,794||774||1|
|5||7.62mm Rifle Lead Core Full Metal Copper Jacket, Military Ball (.308 Caliber)||150 grains||9.70||2,750||3,025||838||1|
|6||9mm Full Metal Copper Jacket with Lead Core||124 grains||8.00||1,400||1,540||427||5|
|Level 7||5.56mm Rifle Full Metal Copper Jacket with Lead Core (.223 Caliber)||55 grains||3.56||3080||3388||939||5|
|Level 8||7.62 Rifle Lead Core Full Metal Copper Jacket, Military Ball (.308 Caliber)||150 grains||9.70||2750||3025||838||5|
Many convenience stores, banks, and buildings with similar security needs opt for security levels 1-3. Purchasers must take weight and cost must come into consideration- especially for schools which are usually on a tight budget. The thicker the bullet resistant glass, the more costly it is to manufacture, transport and install. And like any glass, the framing that supports it plays a major roll as well- another expense. So this begs the question- at what point is bullet resistant glass “thick enough”? What becomes the balance of “safe enough” yet still cost effective? This has become the roadblock for legislation.
Industry experts agree that more education is needed regarding the cost, integration and safety levels of bullet resistant glass so that the governments can choose the best path for protecting America’s schoolchildren.
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