Brian Pfleuger
Moderator Emeritus
All I read is that it must be a weapon designed to be fired from the shoulder, not that it must have a stock attached.
If it doesn't have a stock attached, it's not designed to be fired from the shoulder. Don't make it say things it doesn't say. The receiver is not a rifle or a handgun. It does not meet the qualifications of either. In fact, it can not be "designed to be fired" at all, since it has no barrel.
Here is the ATFs 2011 decision on firearms and parts kits.
Here are their definitions for:
Pistol
Pistol. A weapon originally designed, made, and intended to fire a projectile (bullet) from one or more barrels when held in one hand, and having (a) a chamber(s) as an integral part(s) of, or permanently aligned with, the bore(s); and (b) a short stock designed to be gripped by one hand and at an angle to and extending below the line of the bore(s).
Revolver
A projectile weapon, of the pistol type, having a breechloading chambered cylinder so arranged that the cocking of the hammer or movement of the trigger rotates it and brings the next cartridge in line with the barrel for firing
Rifle
A weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of the explosive in a fixed cartridge to fire only a single projectile through a rifled bore for each single pull of the trigger, and shall include any such weapon which may be readily restored to fire a fixed cartridge.
Shotgun
A weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of the explosive in a fixed shotgun shell to fire through a smooth bore either a number of projectiles (ball shot) or a single projectile for each pull of the trigger, and shall include any such weapon which may be readily restored to fire a fixed shotgun shell.
A receiver clearly does not fit any of those definitions.
Further, on the 4473, the instructions for Question 18 say:
Question 18. Type of Firearm(s): Check all boxes that apply. “Other” refers to frames, receivers and other firearms that are not either handguns or long guns (rifles or shotguns), such as firearms having a pistol grip that expel a shotgun shell, or National Firearms Act (NFA) firearms.
ATF regulations can be confusing and stupid, but this one is at least not confusing. It's crystal clear.