A harmonica gun or slide gun is a firearm design which was breech loaded with a steel slide, containing a number of chambers bored in it that were filled with projectiles. Most harmonica guns are percussion cap guns, although some designs exist for compressed air guns[1][2] and some examples were made in pinfire cartridge form.[3][4] In percussion cap guns, each chamber contains a separate primer, powder charge, and projectile. The slide was inserted in an opening in the breech action and could be advanced by releasing the camlock, and moving the slide by hand. The gun comes in both pistol and rifle models, as well as single-action and double-action.
The earliest example of a harmonica gun is probably the one constructed by the Swiss inventor Welten in 1742.[5]