eCFR :: 27 CFR 478.12 -- Definition of Frame or Receiver.
Firearms History, Technology & Development: Parts of the Firearm: The Receiver
Denver wants to regulate 'ghost guns.' Here's how it might work - Denverite, the Denver site!
New York AG announces cease and desist orders to websites selling ghost guns | 870 AM 97.7FM News Talk WHCU
Firearms - Guides - Importation & Verification of Firearms, Ammunition - Gun Control Act Definitions - Firearm | Bureau of Alcohol, Tobacco, Firearms and Explosives