The Gatling Gun
The Gatling Gun was the predecessor to the modern machine gun.

The inventor of the Gatling Gun was named Richard Gatling. He hoped that the weapon, a deadly one at that, would convince people to stop warring after they saw it. To Gatling's dismay, this never happened. Instead, it only made carnage easier.
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While this weapon was not widely used in the Civil War, it was used by Union forces in the Siege of Petersburg, Virginia, which lasted from 1864 to 1865.