I agree that the current rules don’t make an upper a serial numbered receiver, but can you show me where it says they would have to go through the rulemaking process again to add an additional receiver designation?
Everything I see indicates that the list is non-exhaustive and can be added to based solely on the determination of the ATF. I agree that to add an upper would appear to conflict with what they are claiming to want to do, but it is way less of a stretch than bumpstocks being machine guns.
I literally quoted it. The rule (if adopted) verbatim says that in an AR-15 the lower receiver is the "receiver". They would have to remove that paragraph from the rule to make any other part of an AR-15 the "receiver". The way you do that (remove a paragraph) is the rulemaking process. (notice of proposed rule change, public comment, final rule change, publishing in federal register)
I think a lot of people don't understand what they did with the bump stock thing (which, you will recall, lost in court). If they had a CFR that literally said "A bump stock is not a machine gun" then they couldn't have made it one with a determination letter.
IANAL, but I did spend the last 30 days reading about 600 pages on Laws, CFRs, rulemaking, and regulatory authority because the white house brought it up.
This is only the proposed rule change, so it may change before final adoption, but the rule,
as proposed explicitly says the lower of an AR is
THE[/i] receiver for the purposes of federal law.
The only caveat I see is that they mention if you combine the frames of two different weapons into one firearm, both frames remain firearms in their own right:
(2) Frames or receivers of different weapons that are combined to create a similar weapon each retain their respective classifications as frames or receivers provided they retain their original design and configuration.
This is referring (I suspect) to .50 BMG uppers, since they have previously been determined to be firearms. A "normal" AR upper, as it exists today, is not a "different weapon" then the lower and falls under the spelled out definition I quoted above.
If you were to design some new fangled something that pins to an AR, after the adoption of the new rule you would need to submit that for a determination, because that's not an "AR tybe" firearm. Future versions of the belt-fed upper or AR57 may find themselves a firearm in their own right.