Gatling Gun was inspired by seed-spreading machine and used mainly for domestic intimidation
That said, it's fascinating to read that Gatling's death-machine was inspired by an earlier invention of his, one both mundane and humane — a mechanized seed-sprayer.
One of his agricultural inventions, a seed planter, was the inspiration, Keller believes, for the gun: "Fed by a gravity-driven hopper, the seeds dropped, one by one, into the furrow. Gatling couldn't get that process out of his mind: its rotating simplicity, its smooth mechanical perfection."
The most effective use of Gatling guns during the civil war, Keller writes, was by the New York Times, which lined them up outside its offices to scare away a mob of anti-conscription activists upset with its editor's outspoken criticism of them. They were far more successful among police departments and factory owners than as actual military weapons — as intimidating as they were ineffective, they made perfect security theater.
Jonathan Yardley on 'Mr. Gatling's Terrible Marvel'
[Washington Post]

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I think I agree with Yardley. While the Gatling gun is an interesting subject, fairly little about it seemed to be a "eureka!" design. The Puckle gun came nearly 150 years earlier (1718), featuring a revolver-style action on a flintlock rifle. The Mitrailleuse was already seeing field use France and continued to be used widely through the Franco-Prussian War (1870-1871). Therefore, Gatling's innovation was neither the rotating mechanism itself, nor even the use of multiple barrels to limit overheating (like the Mitrailleuse). Rather, the innovation was largely in the reloading mechanism, which was itself similar in operation to many early combustion engine designs.
Like many great inventions, it was really not so much a pure act of creation as it was an ingenious confluence of ideas.
As an aside, the Gatling design improvement (2.0?) also just barely missed the distinction of being the first self-powered machine gun–that was the Maxim gun.
I believe it ceases to be security "theater" when the threat is real. As was most gruesomely demonstrated at the Ludlow massacre. And I can think of few more successful military inventions. A Gatling gun is still standard equipment on nearly every fighter plane, many helicopters, and is used for anti-missile defense on ships. There's probably one being fired in Afghanistan or Iraq as I type this.
Rooster Cogburn used a gatlin gun to mow down a swath of woods decades before the "Contact!" scene from predator.
(Nobody's got a "Rooster Cogburn" clip on youtube showing Wayne using the gatlin gun, so you'll have to settle for a still shot.)
Gatling Gun != M61 Vulcan. The Vuclan is much more user friendly, and much, much more enemy unfriendly.