As stated in the article:
" 27 C.F.R. 478.11, the definition of firearm frame/receiver states that it is 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.”
As written, neither a Mac style lower receiver nor an AR-15/M16 lower contains all of the features listed. But neither does the upper. It is my understanding that the "mechanism" is basically the recoil spring. So it would seem to me that the problem of the definition has existed since the enactment of the National Firearms Act of 1934. The original model of 1921/28 Thompson sub machinegun has most of those items listed in the definition, in the upper receiver, which is considered the registered part. But the part that strikes the firing pin is in the lower receiver.
A privious Chief of the Firearms and Ammunition Technology Division, complained about the very issue that the lower of the AR-15/M16, which is the registered part, only has half the features listed in the definition 30 years ago. It is my understanding that he testified for the guy in CA accused of having an AR-15 lower receiver build party. Congress makes laws. The BATF&E is part of the Executive Branch, which enforces the laws. This is Congress's problem to fix. The BATF&E can make recommendations to Congress period. If an object must have all of the features listed to be legally considered a "firearm", then there are lots of objects with maker's information and serial numbers that are not legally firearms. The only way that I know of to fix that is for Congress to amendment the definition of "firearm frames or receiver". YMMV.
Scott