TexasRifleman
Moderator Emeritus
When do firearms parts actually constitute a firearm?
The safe bet is to consider the piece with the serial number to be a "firearm" whether it's functional in that condition or not. ATF defines what exactly a firearm is, take for example an AR lower pictured below: That's a firearm all by itself. And, if it's designated by the manufacturer as such, or has been built as such, possibly even a handgun.
All by itself: