The fact that this is a thing.

They work surprisingly well.

A couple years ago there was a story in the news, a girl killed her boyfriend filming a YouTube video. He held a book in front of his chest, and she shot it with a .50 AE Desert Eagle, which is many times more powerful than a 9mm. It went through and killed him. The reason they thought it would work is they shot the book by itself and it stopped the bullet. The difference being when he wasn't holding the book it moved absorbing a bunch of energy.

If it was two books or a smaller bullet it would have stopped it. I know first hand off more than a few gunsmiths that have set up a bullet trap for test firing that is made of stacked phone books.

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