Section 2.1 of the ATF’s National Firearms Act Handbook, titled Types of NFA Firearms states:

The NFA defines the specific types of firearms subject to the provisions of the Act. These definitions describe the function, design, configuration and/or dimensions that weapons must have to be NFA firearms. In addition to describing the weapon, some definitions (machinegun, rifle, shotgun, any other weapon) state that the firearm described also includes a weapon that can be readily restored to fire. A firearm that can be readily restored to fire is a firearm that in its present condition is incapable of expelling a projectile by the action of an explosive (or, in the case of a machinegun, will not in its present condition shoot automatically) but which can be restored to a functional condition by the replacement of missing or defective component parts. Please be aware that case law is not specific but courts have held that the “readily restorable” test is satisfied where a firearm can be made capable of renewed automatic operation, even if it requires some degree of skill and the use of tools and parts.