what part of an ar 15 has the serial number
Receiver (firearms) For the purposes of United States law, the receiver or frame is legally the firearm,[3] and as such it is the controlled part. The definition of which assembly is the legal receiver varies from firearm to firearm, under US law. Generally, the law requires licensed manufacturers and importers to mark the designated receiver with a serial number, the manufacturer or importer, the model and caliber. In addition, makers of receivers are restricted by International Traffic in Arms Regulations. Thus, in the case of a firearm that has multiple receivers (such as the AR-15, which has an upper and lower receiver), the legally controlled part is the one that is serialized. For the AR-15 rifle, the lower assembly is considered the legal receiver;[4] while in the FN-FAL rifle, it is the upper assembly that is serialized and considered the legal receiver.[5][6]