Zip-Gun Bomb
Sometimes considered a type of booby-trap, a zip gun bomb is a type of improvised explosive device, often a mail bomb, that fires bullet(s) instead of detonating explosives when the package is opened. Usually, the bullets or shotgun shells are wired into an electronic circuit, with hot wires set into their powder chambers, thus no firing pin is required to fire the bullets or shells. These devices could be a real problem for postal authorities as they could fall well under the 16oz. rule where a package has to be handed to a postal employee or it's returned. A zip gun bomb could easily be fitted into a dvd case or similar slim package.
Authorities have told the news that the bomb squad was called to disarm a suspicious device after postal authorities x-rayed the package. Bullets, wired into an parallel electrical circuit, were affixed to a base set to be fired in 4 directions when the package was opened. The device was called a "zip-gun bomb".