Yes. In some cases, items being marketed as “unfinished” or “80%” receivers do actually meet the definition of a “firearm” as defined in the Gun Control Act (GCA).
If you are unsure about whether an item you are planning to buy or sell is considered a firearm under the GCA, please contact ATF’s Firearms and Ammunition Technology Division (FATD) by email at fire_tech@atf.gov or by phone at 304-616-4300.