<P> For the purposes of United States law , the receiver or frame is legally the firearm , 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 ; while in the FN - FAL rifle , it is the upper assembly that is serialized and considered the legal receiver . </P>
<P> For the purposes of United States law , the receiver or frame is legally the firearm , 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 ; while in the FN - FAL rifle , it is the upper assembly that is serialized and considered the legal receiver . </P>
what part of an ar 15 has the serial number
what part of an ar 15 has the serial number
lower assembly
lower assembly