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A drum magazine is a type of firearm magazine that is cylindrical in shape, similar to a drum. Instead of rounds being stored flat, as in a more common box magazine, rounds in a drum magazine are stored in a spiral around the center of the magazine, facing the direction of the barrel. A precursor to the drum magazine was the Accles drum, used for Gatling guns. The first true drum magazine was invented between 1908 and 1915 for the Farquhar-Hill Rifle, the patent for which can be found here.
There are several primary designs for a drum magazine. The most common is the cylinder type, that has a spider gear assembly that has an opening in each gear for two to three rounds of ammunition and can be loaded from the rear, used primarily in the AK-47. Another common design is the double-stack dual-horn drum, which operates like a standard stick magazine, but diverges the ammo into two separate feed chutes that run on a single cog. Rimmed ammunition including shotgun ammunition operated drums primarily run off a cogged (gear shaped) design which feeds each individual round of the ammo from the outermost edge of the drum. Recently a single-stack compact design has been released, which uses most of the interior capacity of the drum. It is driven by a single hub and telescopic shaft.
The advantage over traditional box-shaped magazines is that a drum magazine can carry much more ammunition, often two to three times that of a box magazine, such as the 71-round drum for the Russian PPSh-41 submachine gun, without making the gun too big to carry easily. Rear-loading drum magazines, such as the 75-round AK-pattern drums manufactured by Norinco, can be left loaded for extended periods of time without having any tension in the spring. The spring is then wound up when the weapon is ready to fire.
The downside to drum magazines is that they increase the overall weight of the weapon in which they are being used. They are also more prone to jamming due to the complex spring mechanisms they contain. Some drums also have a tendency to rattle when they are loaded with ammunition, possibly giving away the user's position.
The most famous Western examples of a firearm using a drum magazine are the iconic 1930s-era Thompson submachine gun and the presently sold semi-automatic copy which had and have both 50 and 100 round drum magazines available for it. The Thompson also has 20 and 30 round box magazines available, demonstrating the difference in carrying capacity between a box and a drum.
More recently double-drum designs have come into greater use. Where normal magazines put rounds in two rows, in a "double drum" two drums resting on either side of the weapon each hold one row, the two of which combine into one row before entering the receiver. Examples are the World War II era MG 15, and the modern Beta C-Mag. These systems have the advantage of storing even more rounds than a regular drum, while improving the distribution of weight.
The most compact of designs released in the early 1990s was the coil magazine, this particular magazine works with non-protruding rimmed ammunition and is currently manufactured for .308 variants like the M14/M1A1, SR-25, FN FAL and HK 91. The design allows for magazines to stack against each other with no separating walls and uses the geometry of the housing to keep the ammunition pressed against each other in a single stack formation.
The drums of aircraft cannons such as the M61 Vulcan and GAU-8 Avenger resemble drum magazines for small arms, but function in a different manner. That is, the rounds are stored nose-facing-in, and are kept under positive control by partitions running the length of the drum, and are driven forward by an externally powered helical auger. This makes their operation very reliable, even when operated at rates of fire of several thousand rounds per minute.