It's a small explosive charge that you attach to an actor (with a leather vest, a metal plate, or similar protection underneath) and explode to simulate a bullet hit. Often people will attach a condom full of fake blood so that the explosion sprays blood around too.

They're not dangerous to use provided you take all the appropriate precautions, but could permanently maim someone if you don't. So be careful if you do use them.

how much do they cost and where can i get some?

Steve-O
Unknown Name Film Company

As Mark pointed out, a squib is an explosive.

I have a minor disagreement with his statement about the dangers of this device. I'm a licensed pyrotechnician - a squib is dangerous even when used by an experienced professional.

Please, please, please don't use explosives charges, no matter how small, on an actor. Even if you really want to. Even if your movie needs that effect.

A great alternative is compressed air, aquarium tubing and fake blood. With the proper angle and sound effect you can to a great bullet hit with plenty of force and blood without putting any one in danger.

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The aim of an argument or discussion should not be victory, but progress.
Joseph Joubert, essayist (1754-1824)

true, sometimes the effect above looks much better than a squib.

check out this link - http://www.exposure.co.uk/eejit/blood/