GP MG.

This is  coolbet:

"I know it when I see it, I just can't define it."

From that previous blog entry an AR-15 with "bump stock" not ruled by a military appeals court as a machine gun.

As extracted from the wiki and thanks to same:

"A machine gun is an auto-firing, rifled long-barrel autoloading firearm designed for sustained direct fire with fully powered cartridges. Other automatic firearms such as assault rifles and automatic rifles are typically designed more for firing short bursts rather than continuous firepower, and not considered machine guns. Squad automatic weapons, which fire the same (usually intermediate-powered) cartridge used by the other riflemen from the same combat unit, are functionally light machine guns though not called so. Submachine guns, which are capable of continuous rapid fire but using handgun cartridges, are also not technically regarded as true machine guns."

Those main criteria as defining a machine gun:

* ONLY auto-fire. * Hi-power military round [i.e. 7.62 X 51 mm NATO]. * Sustained auto fire.

Automatic weapons such as the Soviet-era RPK or RPD firing an intermediate round [7.62 X 39 mm].

Assault rifles such as the Soviet AK or the American M-16  capable of auto fire [with an intermediate round] but not SUSTAINED fire. 

A fully auto weapon such as the Beretta M12 firing a pistol round and appropriately referred to as a sub-machine gun.

Got it? General purpose machine guns [GP MG] too generally a CREW served weapon. 

NOW you know the rest of the story and are so much the better for it too!

coolbert.