It’s a tiny little box that turns your Glock handgun into a machine gun, and it comes directly to your mailbox courtesy of the internet and factories in China. As it turns out, everyone from neo-Nazis to jihadis in Syria love the thing. The feds? Not so much. U.S. law enforcement has been cracking down on the devices but the cases keep coming. So what’s a Glock conversion kit and who’s buying them?

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From China with love: Court filings from a number of criminal cases show that the feds launched a formal investigation into the conversion switches, which turn handguns into firearms that meet the legal definition of a machine gun, back in February. In charges filed in New York, Missouri, and Illinois, agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives, Customs and Border Protection, Homeland Security Investigations, and the Postal Inspection Service have intercepted packages of the switches they say were purchased through Chinese e-commerce apps. The task force “focused on the importation of switch parts from China that are being used to modify Glock firearms into machineguns,” according to the court documents. Glock doesn’t make machine gun conversion kits, but illicit manufacturers slap the devices with counterfeit Glock logos in order to give them an official appearance.