Definition of Assault Weapons
http://civilliberty.about.com/od/guncontrol/a/Assault-Weapons.htm
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Fully-automatic weapons have been prohibited in the U.S. since the National Firearms Act of 1934. Fully-automatic firearms can spray fire with a single pull of the trigger, while semi-automatic guns fire one shot with each pull of the trigger.
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Shortly after the law was allowed to sunset in 2004, the U.S. Centers for Disease Control and Prevention released a study of the AWB, in which it found “insufficient evidence to determine the effectiveness” of the law for reducing crime rates.