AR-15 lowers are not firearms

https://www.justice.gov/oip/foia-library/osg-530d-letters/9_8_2016/download

AR-15s have split receivers. The upper receiver ordinarily houses the bolt or breechblock and is threaded to receive the barrel. The lower receiver houses the hammer and firing mechanism. In this case, the defendant purchased only a lower receiver. The indictment did not allege that he also possessed or received the upper portion of a split AR-15 receiver-the portion that houses the "bolt or breechblock" and that attaches to the barrel. The government therefore conceded in response to the defendant's motion to dismiss that the lower receiver purchased did "not perfectly fit the CFR section definition." 2016 WL 3556810 at *3.