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 . " Unfinished receivers " , also called " 80 percent receivers " or " blanks " , are partially completed receivers with no serial numbers . Purchasers must perform their own finishing work in order to make the receiver usable . The finishing of receivers for sale or distribution by unlicensed persons is against US law . Because an unfinished 80 % receiver is not a firearm , purchasers do not need to pass a background check . The resulting firearm is sometimes called a " ghost gun " . An AR-15 variant made from an 80 % receiver was used in the 2013 Santa Monica shooting
2013 Santa Monica shooting
2013 Santa Monica shooting
Receiver (firearms)