The latest solicitation for money that I've gotten from the NRA mentioned a "Federal 'Arsenal License'" and describes it as "If you own more than 20 firearms (or parts such as receivers, which are regulated as firearms), or 1,000 rounds of ammunition or primers, you will need a special $300 'federal arsenal license.'" It goes on to say, "To get an 'arsenal license,' you'd have to open your home to inspections by ATF agents three times a year."
ETA: Ah, here's more:
Seems like it doesn't have much traction, fortunately, and has pretty much died. But if enough mall shootings happen, it may provide the ammunition for making more victim disarmament zones...