Also those questions where rhetorical... They were to make a point... They were not serious.
It seems you completely missed my point.
My point was... At what point is it a gun, and at what point is it parts... This is something that must be defined...
Say you have an AR... You want to replace the barrel or stock.. . You can do so easily, as those parts are not considered the "gun"... You do not need a background check to buy a new barrel or stock.
Say a part breaks in the bolt.. You can buy a replacement part easily, as that part is not the" gun".
Also, that such a definition must exist due to criminal law... Without a definition of "what is a gun" a person could have a gun part (say an AR bolt) in their pocket... Get stopped and searched by the police, and could be arrested for "having an illegally concealed firearm". You need the definition to point to, something you can use to ensure the law is followed, and that ambiguity can not creep in (as much as possible) and cause stupid situations like the one I described.
This in contrast to some countries, where on an AR, the upper or barrel is also serialized, and is considered part of the gun, meaning upper swaps are a no go without going through the paperwork that country requires. Some countries have limits on how many you can own, and an upper would still count as a firearm against that limit even though it had no lower and could not function. If you bought the upper and lower separately, it would count as two firearms due to having two different serial numbers.
Yes some firearms makers put the serial number in multiple places, (seems it mostly European makers, who do more business in countries that require such marking) but in US law, it has guidelines on what parts constitute the firearm and are therefore suitable for serializing as the "firearm"... It has to be a main frame/receiver part that holds the action or FCG. So you couldn't mark the barrel and claim that is the "gun".
What I wrote was shorthand for that complicated idea... Where I left out extraneous info in order to prevent confusion. So by "serialized part", I meant "the only serialized part that matters for purposes of the law".