The national firearms act declares a machine gun as:
a. Any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot without manual reloading, by a single function of the trigger.
UNITED STATES v. MILLER
a. As machine guns are "ordinary military equipment" the prohibition of such arms would be unconstitutional.
DISTRICT OF COLUMBIA v. HELLER
a. FOPA is a "prohibition on an entire class of 'arms' that Americans overwhelmingly choose for the lawful purpose of self-defense" and unconstitutional.
The M4 is a standard issue rifle in the US military and its semi-automatic clone, AR15, is one of most popular weapons for self defense.