It only fires once with one pull of the trigger. It just happens to fire more than one chamber. If it were to be classed as a machine gun numerous double barrel shotguns would fit that definition.
Shotguns only fire one shot with the pull of the trigger. My understanding is that if an o/u shotgun was modified to fire two shots at the same time, it would be classified as a machinegun. I've also been told in another discussion that modifying a COP .357 (4-barrel pepperbox pistol) to fire more than 1 shot at a time would make it a machinegun (this was done for the movie blade-runner)
1 trigger pull......more than one shot fired = machine gun. That is how the Beehive works.