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Good Question: What Makes Glass Bulletproof?

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MINNEAPOLIS (WCCO) — An Idaho man has been charged with trying to assassinate the President of the United States by allegedly firing an AK-47-style gun at the White House. But his bullet hit a reinforced White House window that most of us would call “bulletproof.” So how does a piece of glass become bulletproof?

Well, it doesn’t. At least not entirely, according to Bryce Stahlke the manager of Glass Doctor Minneapolis.

“The name for it is bullet resistant glass. There really is no such thing as bulletproof,” said Stahlke.

Bullet resistant glass is an engineering combination of layering and thickness.

“This piece is multiple layers of glass and liquid laminate sandwiched together,” he said showing a piece of level 4 glass, strong enough to stop three shots from an AK-47.

It weighs about 300 pounds, and is 2 ¼ inches thick.

“I count 9 layers of glass and 8 layers of lamination,” said Stahlke.

The physics of the operation is fairly simple.

“The hardness of the glass flattens the bullet. The lamination slows it,” he said.

That slowing process is the key, so the bullet doesn’t pierce the multiple layers. After a shot, bullets are often lodged in the glass as smashed, flat hunks of steel.

Some form of bullet-resistant glass has been around since the early 1900′s. During World War I and II, fabricators used liquid rubber between panes of glass.

There are Minnesota fabricators of bullet-resistant glass and two manufacturers near Wausau, Wis.

“There are other configurations you can do with glass and polycarbonate,” a plastic material which is stronger than plexiglass, said Stahlke,

To replace the glass of a typical large window would cost $300, according to Stahlke, but to replace an identical size with level 4 bullet-resistant glass could cost up to $2,000.

Different levels of glass stop different types of ammo. Level 1 glass is rated to stops three rounds of a 9-millimeter handgun.

“This won’t stop much of a bullet; this will stop a person,” said Stahlke.

Level 2 glass stops a .357-Magnum bullet. Level 3 stops a .44-bullet about twice as fast as a 9-mm weapon. Level 4 stops the AK-47.

“It’s very expensive. Most of the bulletproof glass we quote isn’t installed,” he said.

You’ll find the special glass in police stations, government buildings and banks.

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