"Depending on specific circumstances. If the manufacturer puts a serial number on it, and lists it as a firearm, whether 80% finished or 0% finished, it is legally, from that point on, a firearm"
From a practical standpoint, and germane to the OP's question, NOBODY is selling 80% machined lowers that have been listed by a Type 7 FFL as a manuctured firearm and then sold as a firearm. AFAIK nobody ever has. 80% lowers are made and sold to be finished (and serialized) by the manufacturer of a firearm, either a Type 7 FFL or an Individual. If you want to machine out the pocket to accept an airsoft drop in mechanism and to "serialize" it so that it looks authentic, it's perfectly legal.