Criminals in the United States are using 3D printers to manufacture small plastic devices converting semi-automatic rifles into illegal machine guns that can fire hundreds of rounds per minute with a single pull of the trigger, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), a US federal agency regulating firearms. William Ryan, chief of the firearms and ammunition technology division at ATF, says the agency is working to warn local police agencies about so-called machine gun conversion devices as they are increasingly used to carry out crimes. "We're seeing them turn up in crimes and on the streets much more often," Ryan says. IMAGES AND SOUNDBITES