No New Legislation Required!
It would appear, according to USC › Title 26 › Subtitle E › Chapter 53 › Subchapter B › Part I › § 5845, that they are already classified as machineguns:
(b) Machinegun
The term “machinegun” means 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. The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and 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.
The DOJ & ATF merely need to treat them as such.
Since a simple Google search returned this interesting page, I'm sure that most assault-rifle style autoloading rifles would meet the can be readily restored criterion.