Keir B. Sterling, "Gatling, Richard Jordan," American National Biography Online, February 2000, http://www.anb.org/articles/13/13-00589.html.
With the coming of the Civil War, Gatling began working on improvements in weaponry. His first inventions in this area were a marine steam ram and a rapid-fire, or machine gun, with which his name has ever since been identified; both were patented in 1862. The gun may have been invented because of a suggestion of Colonel R. A. Maxwell that the Union army would require just such a specialized weapon during the coming war. Before the end of that same year, he had produced a working model of this weapon, a hand-cranked .58 caliber version with six barrels, rotated around a central axis. Cartridges dropped from a drum atop the gun and were automatically fed into each barrel. The rate of fire was 350 rounds per minute, but there were problems with accuracy, and the weapon sometimes jammed when fired too rapidly. This was owing in large part to the use of the then-common paper cartridges.