Here's your answer:
Any weapon which shoots automatically more than one shot, without
manual reloading, by a single function of the trigger meets the
definition of a machinegun in section 5845(b) of the National
Firearms Act (NFA). An AR-15 rifle, which is assembled with
certain M-16 machinegun fire control components, and which is
capable of shooting automatically is a machinegun as defined.
The definition of machinegun in section 5845(b) also includes any
combination of parts from which a machinegun can be assembled if
such parts are in the possession or under the control of a person.
Thus, an AR-15 rifle possessed with separate M-16 machinegun
components can meet the definition of a machinegun, if the rifle
shoots automatically when the components are installed.