Putting Las Vegas In Perspective

There was something called a "Puckle Gun" that was called a machine gun in 1718, but it wasn't automatic, each cylinder had to be advanced by hand.

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There were multi barrel pistols such as the Garrucha (1730) and stuff like this:

.Hand operated weapons such as the Gatling Gun didn't arrive until 1861.
But the first automatic machine gun that used it's own recoil to operate was the Maxim Gun, 1883.

The Constitution was signed in 1787.
So 6-eyed is correct there were multi barreled weapons of limited use and practicality, but not automatic machine guns.