The bullet resistant windows in convenience stores and banks are often made of cell cast acrylic sheet or a combination of acrylic and Polycarbonate. They are often about 1.25" to 1.375" thick and are designed to protect against threats that are likely to be encountered in that environment. Typical bullet resistant ratings of UL.752 Level 1 to Level 3 are encountered. But what does a UL.752 Level 1, Level 2 or Level 3 mean and how does it compare to the transparent armor of military applications?
A UL.752 Level 1 material is designed to stop 9mm FMCJ rounds weighing 8.0 grams traveling at a velocity of up to 394 meters/second.
A UL.752 Level 2 material is designed to stop 0.357 Magnum JSP rounds weighing 10.2 grams traveling at a velocity of up to 419 meters/second.
A UL.752 Level 3 material is designed to stop 0.44 Magnum rounds weighing 15.6 grams traveling at a velocity of up to 453 meters/second.
But what does this mean? One of the most important factors in determining whether a bullet resistant structure will stop a ballistics round is how much Kinetic Energy does the ballistics round have.
Using the equation for Kinetic Energy:
Kinetic Energy (Joules) = 1/2 x Mass (Kilograms) x Velocity (meters/second)^2
Calculating the Kinetic Energy for the UL.752 Level 1 ballistics round we find:
Kinetic Energy = 1/2 x 0.008 x 394 x 394 = 620 Joules
For the three UL.752 Levels we get:
Level 1 620 Joules
Level 2 895 Joules
Level 3 1600 Joules
We can see as the weight and the velocity of the round increase the Kinetic Energy of the round increases. The bullet resistant material needs to be able to resist a larger amount of Kinetic Energy.
We can now look at the military grades to compare the amount of Kinetic Energy they are designed to stop. Military grades of transparent armor are composed of multiple layers of glass and polycarbonate. The glass can be of various types. In some cases advanced materials such as Spinel and ALON are also used. Often the structures can be many inches thick.
For US military grades a standard known as ATPD.2352 is used. The different rounds that the materials must stop is listed but the velocities are classified. The fact that the velocities are classified makes it difficult to calculate the required Kinetic Energy that must be absorbed; it would be possible to take an educated guess at the velocities, but for the purposes of this blog post, we do not need to do this is we can use the NATO standard AEP55 STANAG 4549 Volume 1.
STANAG 4549 has 5 protection levels for Light Armored Vehicles. For the purposes of the discussion on transparent armor we will just look at Levels 1 and 4.
Level 1 material is designed to stop a 7.62 mm x 51 NATO ball round weighing 9.65 grams traveling at 833 meters/second.
Level 4 material is designed to stop a 14.5 mm x 114 API/B32 round weighing 64 grams traveling at 911 meters/second.
A Level 1 round has a Kinetic Energy of 3,348 Joules
A Level 4 round has a Kinetic Energy of 26,557 Joules
You can see that the energy that a UL.752 Level 1 material needs to stop is over 40 times less than a STANAG 4549 Level 4 material. The reason for this difference is that the type of ballistics rounds likely to be encountered at a convenience store are likely to be very different from those encountered by the military. Indeed the deterrence factor of bullet resistance glass in commercial applications should not be underestimated.
It should be noted that this discussion is very much a simplification and is only meant to compare the Kinetic Energy of the different rounds used for the different tests. There are a number of parameters that have not been discussed in this blog post such as the multi shot spacing and the shape of the round.