The Colt revolver was the most popular weapon for self defense of the 1850's. The Colt company actually made the Gatling Guns, and the colt revolvers rotating cylinders have a clear similarity to the revolving barrel idea of the Gatling Gun.
The "Coffee Mill Gun" was a gun made during the civil war, and despite denial from Richard Gatling had the exact same loading system as the Gatling Gun, which both had many malfunctioning errors during the civil war. This gun however close to the Gatling Gun didn't have revolving barrels and shot at an average of 120rpm vs the Gatling Guns 400rpm on the 1865 model. The Gatling Gun replaced this gun.
The Nock Volley Gun, a navel warfare weapon, was thought to have influenced the creation of the Gatling Gun, because it was made 82 years before in 1780, and used multiple barrels for more firepower.
Later in history the idea of multiple revolving barrels like the Gatling Gun was reused in Project Vulcan. Project Vulcan was a project by the government to make faster firing guns to make aircraft to aircraft combat more effective. When they used the multiple barrel idea the could make weapons firing up to 6000rpm. The product of Project Vulcan was the m61 machine gun.