I think this whole AR15 thing is going to blow up in our faces. ATF has to make decisions within the realm of the idiotic laws Congress has passed (and regulations that ATF has made in the past that might have made sense at the time) and at some point they had to decide which part of the AR15 was the frame or receiver. The upper met part of the legal definition and the lower met part of the legal definition. They decided that since the lower was the part that was most different between a semi-auto AR15 and a full-auto M16 that part was the sensible choice. On the original Colts there were some minor differences in the upper, but they were trivial compared to the differences in the lower.
At some point someone is going to get busted with a bunch of home-converted receivers that are milled and drilled for an auto-sear (effectively making them M16 machine gun lowers) and ATF is going to be unable to prosecute them for it because they don't meet the legal definition of a receiver, and then someone is going to decide that something must be done. I suspect that whatever is done will not make our lives easier.
One of the great things about the AR15/M16 design is the ease of changing uppers which allows for simple barrel length and caliber changes. I could see ATF deciding that the upper and lower are married, both must be serialized, and both must match. Then all of a sudden one of the great features of the platform becomes a mess.
With many (most?) of the other guns that have an upper and a lower, the upper is treated as the receiver. FN-FAL, Uzi, HK MP5, 91, 93, 94, G3, etc. So full-auto Uzi, HK, etc. trigger packs are not a regulated thing at all because the upper is the firearm.
After all these years and with all the gajillions of unserialized AR15/M16 uppers in existence I don't think ATF could reclassify the upper as the receiver, and honestly it doesn't meet the definition any better than the lower does.
27 USC 478.11 (in relevant part):
Firearm frame or receiver. 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.
"Firearm frame or receiver" is not defined in the statutory law enacted by Congress - only in the Code of Federal regulations which are rules promulgated by the executive branch agencies charged with administering various statutes. So this is something ATF/DOJ could "fix" without Congress getting involved. They would just have to do a notice of proposed rule making, go through the comment period, etc., just like they did with bump stocks and the CLEO signoff, etc.
It doesn't seem like it would be very hard to fix this without making a mess of the world. Perhaps rewrite 478.11 to read as follows:
Firearm frame or receiver. 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. When a firearm design utilizes a multi-part receiver, and no single part meets the foregoing definition, ATF Firearms Technology Branch will classify one part as the "frame or receiver" and publish such classification in [insert publishing mechanism here] and that part shall be considered the frame or receiver for that firearm design for the purposes of this section.
We'll see what the future holds, but at some point there will be a fish big enough that ATF/DOJ can't handling having to let them off the hook and something will be done about this. Meanwhile, stocking up on unserialized uppers might not be a terrible plan. :-)