A harmonica gun or slide gun is a form of firearm which was breech loaded with a steel slide, containing a number of chambers bored in it and which were filled with projectiles. Most harmonica guns are percussion cap guns. 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, moving the slide by hand. The gun comes in both pistol and rifle models, as well as single-action and double-action.