How Bullet Resistant Glass Breaks

A modern bullet is a very small, very light, very smooth thing moving very fast. When it strikes a barrier, that barrier must absorb the bullet’s momentum or the bullet will crack straight through.

In the following video you’ll see three slow-motion examples of glass being shot. The first sheet is standard plate glass (the glass used in most home windows), the second is tempered glass (used in automobile passenger windows and as part of some bullet proof systems), and the final piece is a sheet of bullet resistant acrylic (which the host refers to as “bullet proof glass.”)

SHOOTING PLATE GLASS

In terms of basic ballistics, firing a gun at a sheet of plate glass is very instructive. A 9mm bullet has a mass around 7.5 grams (about the same as a freshly minted penny and nickel; the U.S. Mint specifies that, at production, a nickel is 5 grams and a penny is 2.5 grams). These 7.5 grams of lead are moving at an average of 1,234 feet per second (i.e., 841 mph). Instead of being cast as a blob or ball, the lead is formed into a smooth cone designed to cut through a resisting fluid (in this case air) without being knocked off its path. When it comes into contact with a brittle, rigid surface–like glass–it transfers very little of its momentum to the surface, and instead pops through, like the little straw popping into a juice box.

This is precisely what we see in the video at around the 0:40 second mark (and repeated in slow-motion at 0:48). The glass doesn’t move at all, and the bullet passes straight through.

TEMPERED GLASS

Tempered glass is four or five times stronger than plate glass. Although the sheet used in the above video clearly is not bullet resistant glass, the earliest “bullet proof” windows were indeed made from stacks of laminated glass. Even today tempered glass is occasionally used in bullet resistant systems.

As you’ll see at 1:15 in the video (and at reduced speed at 1:29), tempered glass breaks very differently from plate glass. Where-as the light, fast bullet pops right through plate glass, it utterly destroys the tempered glass, reducing it to many small cubes. This is by design: Plate glass tends to break into large dagger-like shards. In a storm, accident, or explosion, these can be more dangerous than whatever caused the window to break in the first place. Conversely, tempered glass is less likely to break to begin with, and when it does, the numerous little glass nuggets of shattered tempered glass are unlikely to cause loss of life or limb. Owing to these two characteristics–strength and shattering to particles–tempered glass is also called “safety glass,” and often used in public entryways, display windows, exterior windows on tall buildings, and in the passenger windows of cars.

THE PHYSICS OF TEMPERED BULLET RESISTANT GLASS

There is some neat physics behind this. Tempered glass begins its life as standard plate glass, which is cut to size (it can’t be cut after tempering), then precisely heated and cooled in a specialized annealing furnace. This forces the surface of the glass to cool faster, compressing the still-molten interior. As the interior cools, it pulls against this hardened compressive envelope. This balance of compressive (squeezing) and tensile (pulling) forces makes the glass very strong. But when the surface is broken, these stresses are knocked out of balance, the crystalline structure collapses, and the glass crumbles. (For a little historical tempered-glass trivia, check out this earlier blog post about Prince Rupert’s Drops).

SHOOTING TEMPERED GLASS AT ONE MILLION FRAMES PER SECOND

These balanced stresses mean that a sheet of tempered glass eats up more of a bullet’s momentum upon impact. Early WWII-era bullet resistant glass was made by laminating together layers of tempered glass; a bullet might shatter the first couple layers of this “glass sandwich,” but that shattering would slow the bullet enough to keep it from cracking through the barrier.

Here is some really striking one million frame-per-second super-slow-mo footage of bullets striking tempered glass.

SHOOTING BULLET RESISTANT ACRYLIC

At the 1:52 mark we finally see a legitimate piece of bullet resistant glass get shot. This is sheet of monolithic acrylic (the shot is repeated in slow-mo at 2:16). You’ll note that this true bullet resistant glass behaves very differently from either of the other kinds of glass. The thick, solid acrylic stops the bullet in its tracks. Sharp-eyed viewers will see the acrylic jump back a little: since the bullet slams into the acrylic–rather than popping through–the bullet resistant glass is obliged to absorb all of the bullet’s forward momentum. Because the shot is from such close range and dead on, the bullet doesn’t even ricochet. Instead, its energy is converted to heat by the impact, and the bullet melts. The boiling lead then spatters away as tiny droplets, which splash across the acrylic, marring its surface. (In this section of the one-million frame-per-second super-slow-mo footage you can see bullets actually boil and shatter as they hit lead blocks).

SHOOTING BULLET RESISTANT POLYCARBONATE

The other popular bullet resistant glass, vital to higher-level bullet proof systems, is polycarbonate. In this video, Adam Savage and Jamie Hyneman of Mythbusters explore a number of common “bullet proof” myths (e.g., Can a deck of cards stop a .22? What about a lighter? A Bible?), including their own perception that the quarter-inch polycarbonate they use in their blast shields is bullet proof (spoiler alert: it isn’t).

Of most interest to us is the last half of the video, where they shoot at an actual Level 3 bullet resistant polycarbonate box. This starts around the 5:00 mark. At 5:35 they’ve just shot the box with a .357 Magnum. Rather than boiling away or ricocheting, the bullet is encased in the thermoplastic. This is because the polycarbonate is significantly softer than acrylic, allowing the bullet–which is designed to penetrate–to push underneath the surface of the polycarbonate. The polycarbonate behaves like a very thick fluid offering a tremendous amount of resistance, and successfully sapping the bullet of its energy.

Having stopped a .22, .357 Magnum, and .44 Magnum, the Mythbusters pit their Level 3 bullet resistant glass box against a .30-06 at 6:32. Level 3 bullet resistant glass is rated to resist several shots from a 9mm, .357, or .44–and can often even stop bullets from an M16 or AK-47–but you need Level 4 glass to stop a high powered rifle. And as you see in the video, that .30-06 bullet pops through the Level 3 polycarbonate just as easily as the .38 popped through the plate glass in the first video. Host Jamie Hyneman marvels that the box “didn’t even jump.” This indicates that almost none of the bullet’s momentum was transferred to the bullet resistant box. Because the box couldn’t absorb the forward energy, the bullet cracked straight through. Compare this to the .44 shot at 6:08: The box easily caught the bullet, sliding back to absorb the bullet’s momentum.