I don't even expect a grandfather clause in this one.
How would a "grandfather clause" even work? As I understand it, the proposal would apply at the commercial level, by requiring unfinished receivers to have serial numbers and go through the FFL system. Existing private ownership is not addressed, except in the event of resale. Maybe I'm wrong here.
There are various stages to the process: (1) the fabrication of the unfinished receiver by a manufacturer, (2) the sale of such receiver to a consumer, (3) the completion of the receiver into a working gun, and (4) the possible resale of the working gun to someone else. Which of these stages would be targeted by the proposal?
The way I see it, a person might buy an unfinished receiver for either of
two purposes: (1) to assemble it into a working gun, or (2) to assemble it into a dummy display collectible. There are thousands of dummy display
machine guns out there, made in exactly this way. If the dummy receivers are suddenly deemed to be "guns," then we will have thousands of instant NFA violations. In fact,
any unfinished AR-15 receiver could be deemed an NFA violation, since the "third hole" could be drilled very easily while milling out the firing group pocket.
Either a piece of metal is a gun, or it is not. If we're going to start labeling precursors as "guns," where do we draw the line? Any hunk of aluminum or steel could be a "gun." The existing rules (so-called "80%") were an attempt to draw a line. Flawed as these rules are, messing with them opens a can of worms.