The Breaking Bad series finale aired (originally) Sunday, Sept. 29, 2013. The Examiner's server/s may have issues displaying the Breaking Bad video for this article. The reader should be able to view the "auto-run gadget" for the show's M60 machine gun in the finale with this BB link. Series creator Vince Gilligan explains in the clip (about 2:30 minutes in) about "purposing" the previous purchase of the M60 by Walter White - then re-purposing it for the finale. Those in the know can find out how Walter constructed the self-firing mechanism on the BB website.
The link to the final episode story line is here once again. Walt's construction of the auto-run self-firing machine gun gadget follows right after poisoning (!) Lydia. The BB online story-line explains that Walt makes the auto-run machine gun "rig" out of garage door opener parts and a swivel chair base. The idea and the video images are impressive. The "push-button key fob" activates the rig and causes it to move (and fire) in a sweeping action. Details for the triggering mechanism connection aren't fully apparent. The M60 operator's manual also makes some brief appearances with the weapon.
As to "general machine guns" some sources cite the "Fyodorov" as being the first. The Engineer cites the "first self-firing machine gun" as being the invention of Sir Hiram Maxim circa 1884. Is the auto-run machine gadget legal in the United States? One should probably refer to the U. S. Gun Control Act of 1968. Article 921 section 15 defines a 'fugitive from justice' - somebody like Walter White would likely meet this definition and possession of his M60 auto-run machine gun is likely illegal. The self-firing gadget is likely to be defined as a "destructive device" and likely illegal. As a "booby-trap" device such things are usually highly illegal. The self-firing auto-run M60 may also be defined as a possible "weapon of mass destruction"!
Of course, Walt continues with his "reign of terror and vengeance" after these scenes of brilliantly evil devices. The series ends with both tears of joy and sorrow after Walt dispatches the final pack of bad guys with the auto-run machine gun. Of course, many of you are more expert about the operation and tech specs of the M60 than can be reported in this space. It is sincerely hoped that the experts will not be offended!