18 USC 922(b)
It shall be unlawful for any licensed importer, licensed manufacturer, licensed dealer, or licensed collector to sell or deliver—
(4) to any person any destructive device, machinegun (as defined in section 5845 of the Internal Revenue Code of 1986), short-barreled shotgun, or short-barreled rifle, except as specifically authorized by the Attorney General consistent with public safety and necessity
27 CFR 478.98
The sale or delivery by a licensee of any destructive device, machine gun, short-barreled shotgun, or short-barreled rifle, to any person other than another licensee who is licensed under this part to deal in such device or firearm, is prohibited unless the person to receive such device or firearm furnishes to the licensee a sworn statement setting forth
(a) The reasons why there is a reasonable necessity for such person to purchase or otherwise acquire the device or weapon; and
(b) That such person's receipt or possession of the device or weapon would be consistent with public safety. Such sworn statement shall be made on the application to transfer and register the firearm required by Part 479 of this chapter. The sale or delivery of the device or weapon shall not be made until the application for transfer is approved by the Director and returned to the licensee (transferor) as provided in Part 479 of this chapter.
This part of the law is why you have to state a reason on form 4 transfers. The "reasonable necessity" and "consistent with public safety" language sounds a lot like the language used in may-issue CCW laws. Could the ATF or the attorney general someday interpret the law to restrict NFA transfers by narrowing the view of what is consistent with public safety and necessity?