>>601265
Imagine you take a unibody car, gut it, and slap it down on a rail buggy as a body kit. Then the DMV comes along, and says that since either constituent part could be used to build a complete car, that you need to have two titles and two sets of registration to be legal. As far as they care, you're driving two cars simultaneously. That's the kind of stupid we're dealing with.
The lower receiver is the serialized part on an Armalite because that's the arbitrary decision they made in 1960-something and that's the way it's always been. You can use the same argument to say that the upper should be the serialized part because it contains all of the critical pressure bearing components, namely the barrel, the bolt, bolt carrier, and locking surfaces; the lower is just there for the magazine well the the trigger group.
Furthermore, there are other uppers that don't make use of the magwell on the lower. That one belt-fed upper and the one that takes p90 mags, just off the top of my head. If 'the lower is only used as a trigger group', then doesn't that make them firearms as well?