Under the US Code of Federal Regulations, a firearm frame or receiver is defined as: “That part of a firearm which provides housing for the hammer, bolt or breechblock, and firing mechanism, and which is usually threaded at its forward portion to receive the barrel .”
So when purchasing a lower it is only half of that combination and therefore shouldn’t need the form 4473.
This is the same thing I posted a topic on as well. It’ll be interesting to see how this is handled
ARs are not the only designs that split things up.
I never could understand why the AR lower was the serialized component on an AR, but the upper was the serialized component on an FAL.
Then there’s the issue of HK trigger packs being registered as full-auto firearms by themselves, while they are housed in a lower and the serialized component of the firearm is the upper.
A person could probably write a book on the inconsistencies, but the ATF makes it easier by occasionally changing it’s mind and completely reversing an earlier decision.