During the 1880s the Gatling Gun was manufactured in Hartford, and by the end of the century the company had been absorbed by Colt's Patent Fire Arms Mfg. Co. -- where Hank Morgan "learned his trade," as he puts it. It is presumably a version of the Colts Revolver he uses in the joust with Sir Sagramore. In the battle at the novel's end, sixteen Gatling Guns are used. (Hank's one real invention in the course of the novel is the "Miller Gun"; it shoots money.)