Are some items being marketed as non-firearm unfinished” or “80%” receivers actually considered firearms?

Yes, in some cases, items being marketed as unfinished or “80%” receivers do meet the definition of a “firearm” as defined in the GCA. Persons who are unsure about whether an item they are planning to buy or sell is considered a firearm under the GCA should contact ATF’s Firearms and Ammunition Technology Division (FATD).