This photo provided by the U.S. Department of Justice shows AR-15 lower receivers, which federal agents have seized, including these unfinished ones taken in 2014 in California, for firearms investigations nationwide. For decades, the federal government has treated the mechanism called the lower receiver as the essential piece of the semiautomatic rifle, which has been used in some of the nation's deadliest mass shootings. But some defense attorneys have recently argued that the part alone does not meet the definition in the law. (U.S. Department of Justice via AP)
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