AlexanderA
Member
"80%" is a meaningless figure, that somebody cooked up for marketing purposes. ATF has ruled that when certain machining operations are left undone, an incomplete lower receiver is not a "gun." The issue of whether those undone machining operations amount to 20% of the total machining time, was not addressed by the ATF.80% means, 80% of the manufacturing is already done, (you) do the rest.
Originally, as I understand it, the ATF ruled that if any one of three specific areas of the lower was not machined, the lower would be a "non-gun." These three areas were (a) the magazine well, (b) the socket for the buffer tube, or (c) the firing control group pocket. Today, all the incomplete lowers on the market leave the FCG pocket unmachined. An industry has arisen specifically to provide jigs and tools so that users can machine this pocket easily at home.
There is no federal requirement that such a lower, completed at home for personal use, be marked with a serial number or manufacturer data. Hence, these are untraceable "sterile" weapons, or "ghost guns."
That's the state of the law currently. It's easy to see that the proliferation of such incomplete receivers, combined with the tools to complete them and the unregulated remainder of the parts, has made the existing FFL regulatory system practically moot (at least regarding the AR platform). This may be good or bad, depending on your point of view.