Your conclusion that it is a “stupid ruling” is wrong on its face. The ruling is absolutely correct. The stupidity is that the wrong thing is being serialized and regulated on guns of this type. The things that actually hold, lock the ammunition in place, and ignite it, are the barrel, the breech, and the firing mechanism, regardless of where they are. In the case of semiautomatic weapons, that usually means the barrel and chamber, the chamber locking mechanism, and a firing mechanism, whether a firing pin or some other means of firing the propellant, regardless of whether it is struck with a hammer, ignited with fire, or electrical spark, or other means. The human operated trigger mechanism is essentially irrelevant unless it is legally regulated between semi and full automatic firing and in fact can be located remotely from all of what makes a gun fire a projectile down range. Is those parts that should have the serial number that can be traced.
In the case of the AR-15, that would be the UPPER receiver which holds the bolt, firing mechanism, gas block and tube, barrel, etc., not the lower receiver which only holds the trigger assembly group, magazine, and recoil spring, with shoulder stock and handgrip, all of which are mere incidentals to sending a bullet down range.
Right.
But the sear and whatever parts that make the rifle automatic or semi-automatic are in the lower. What a dilemma for the ATF! May their suffering never end!