John Zawahri's failure to pass a background check prevented him from buying a firearm in California, so the 23-year-old obtained an “unfinished receiver,” the metal piece that holds the critical mechanisms that allow guns to fire, and built an assault rifle himself, says the Washington Post. Last summer, he went on a rampage at a college in Southern California, firing 100 rounds and killing five people before police fatally shot him. Zawahri's assault became one of the most notorious cases involving unfinished receivers, which are unregulated and are available for purchase online and at some gun stores. Officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives view the spread of the receivers as an effort to get around strict gun-control laws, particularly in California.
They acknowledge that they have no idea how many of the components have been made and sold. “That is the million-dollar question,” said Joseph Riehl, agent in charge of ATF's San Francisco office. “We know for sure there are tens of thousands, just in California.” The sale of unfinished receivers, also called “blanks” or “80 percent lower receivers,” is one of the most daunting challenges for law enforcement officials tasked with enforcing firearms regulations. There are no sales records of unfinished receivers, as there are for ordinary gun sales, which means the ATF cannot check with stores for information about buyers when a gun is used in a crime. Because the receivers bear no serial numbers or other markings that would indicate where they were manufactured, guns made with them can't be traced back to their owners if they are found at a crime scene.