The machine gun used in the movie seems to be a (non-existent) combination of two actual machines guns of the time. The multiple rotating barrels design is based on the familiar Gatling Gun, which was a manual crank driven machine gun that used a top-feed, gravity driven clip or drum. In the movie, it's portrayed as an automatic firing, belt-fed machine gun. However, the only automatic firing, belt-fed machine gun of the time was the Maxim, which was a single barrel firearm.