The ATF Firearms Technology Branch examined these a few years back and called them simply a part or accessory.

The complete definition of a "machinegun" under the NFA also includes 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.

After the Las Vegas mass murder, the President ordered them to be re-evaluated. The ATF attorneys got more involved this time around and determined that during the original evaluation the law was not being interpreted correctly. Specifically, the meaning of "single function of the trigger."

After a period of review, of which the public was free to submit their own opinions, they were banned.

I don't agree with their assessment, and believe that it is only going to cause headaches down the road. But, it is what it is.

We need to be diligent because the left will never quit or give up. Every day they are finding more and more creative ways to chip away or end arounds at not just firearms ownership but hunting as well.