Johnson rifle also uses a rotary magazine. The design is still used in some modern firearms, most notably the Ruger American series, the semi-automatic Ruger 10/22, the bolt-action Ruger 77/22 and the Steyr SSG 69. Drum Drum magazines are used primarily for light machine guns. In one type, a moving partition within a cylindrical chamber forces loose rounds into an exit slot, with the cartridges being stored parallel to the axis of rotation. After loading of the magazine, a wound spring or other mechanism forces the partition against the rounds. In all models a single staggered column is pushed by a follower through a curved path. From there the rounds enter the vertical riser either from a single or dual drums. Cylindrical designs such as rotary and drum magazines allow for larger capacity than box magazines, without growing to excessive length. The downside of a drum magazine's extra capacity is its added weight that, combined with the gun, can affect handling and prolonged use. Drum magazines can be more difficult to incorporate into combat gear compared to more regular, rectangular box magazines. Many drum-fed firearms can also load from conventional box magazines, such as the Soviet PPSh-41 submachine gun, RPK light machine gun, and the American Thompson submachine gun. The term "drum" is sometimes applied to a belt box for a belt-fed machine gun, though this is just a case that houses a length of ammunition belt, not a drum magazine. Saddle-drum Before WWII the Germans developed 75-round saddle-drum magazines for use in their MG 13 and MG 15 machine guns. The MG 34 machine guns could also use saddle-drum magazine when fitted with a special feed cover. The 75 rounds of ammunition were evenly distributed in each side of the magazine with a central feed "tower" where the ammunition is fed to the bolt. The ammunition was fed by a spring force, with rounds alternating from each side of the double drum so that the gun would not become unbalanced. Pan The pan magazine differs from other circular magazines in that the cartridges are stored perpendicular to the axis of rotation, rather than parallel, and are usually mounted on top of the firearm. This type is used on the Lewis Gun, Vickers K, Bren Gun (only used in anti-aircraft mountings), Degtyaryov light machine gun, and American-180 submachine gun. A highly unusual example was found on the Type 89 machine