Short answers, pulled from the story.
The Gatling gun was invented by Richard Jordan Gatling of North Carolina in 1861 and patented on the 4th of November, 1862. Gatling stated he designed it to reduce the size of armies and lower deaths from both combat and disease.
A Gatling gun uses six rotating barrels (some models had up to ten) driven by a hand crank around a central shaft. Each barrel fires once per revolution at a fixed position, then ejects the spent casing and cools before rotating back to receive the next round from a gravity-fed hopper. By 1886 the gun could fire more than 400 rounds per minute.
The Gatling gun was first used in combat during the American Civil War. Twelve guns were purchased by Union commanders and deployed in the trenches of the Siege of Petersburg, Virginia, which ran from June 1864 through April 1865. Eight more were mounted on gunboats.
Lieutenant John "Gatling Gun" Parker commanded a separate detachment of four Model 1895 ten-barrel Gatling guns during the 1898 Spanish-American War. During the assault on San Juan and Kettle hills, three of those guns fired 18,000 rounds in minutes, averaging more than 700 rounds per minute per gun, supporting the American advance.
All models of Gatling guns were declared obsolete by the U.S. military in 1911, after 45 years of service. The self-powered Maxim gun, patented in 1883, and lighter tripod-mounted weapons had by then superseded the manually cranked Gatling.
After World War II, the rotating-barrel principle was revived for aircraft weaponry because reciprocating-bolt autocannons were more prone to jamming under high-g conditions. This led to the Minigun and the M61 Vulcan, and ultimately to the 30mm GAU-8 Avenger autocannon mounted on the Fairchild Republic A-10 Thunderbolt II.