A drum magazine is a type of firearms magazine

Magazine (firearm)

A magazine is an ammunition storage and feeding device within or attached to a repeating firearm. Magazines may be integral to the firearm or removable . The magazine functions by moving the cartridges stored in the magazine into a position where they may be loaded into the chamber by the action...

 that is cylindrical in shape, similar to a drum

Drum

The drum is a member of the percussion group of musical instruments, which is technically classified as the membranophones. Drums consist of at least one membrane, called a drumhead or drum skin, that is stretched over a shell and struck, either directly with the player's hands, or with a...

. 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.

There are several primary designs for a drum magazine. The most common of which cylinder drum type that has a spider gear assembly that has an opening in each gear for 2-3 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 shoots that run on a single cog. Rimmed ammunition

Rim (firearms)

A rim is an external flange that is machined, cast, molded, stamped or pressed around the bottom of a firearms cartridge. The rim may serve a number of purposes, the most common being as the place for the extractor to engage...

 including shotgun

Shotgun

A shotgun is a firearm that is usually designed to be fired from the shoulder, which uses the energy of a fixed shell to fire a number of small spherical pellets called shot, or a solid projectile called a slug...

 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 shell. Of recent a single stack compact design has been released for 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

PPSh-41

The PPSh-41 was a Soviet submachine gun designed by Georgi Shpagin as an inexpensive, simplified alternative to the PPD-40. Intended for use by minimally-trained conscript soldiers, the PPSh was a magazine-fed selective-fire submachine gun using an open-bolt, blowback action...

 submachine gun, without making it too big to be impractical to carry. The downside to drum magazines is that they increase the overall weight of the weapon in which they are being used, and they are more prone to jamming.

The most famous examples of firearms using a drum magazine are the iconic 1930s-era Thompson submachine gun

Thompson submachine gun

The Thompson is an American submachine gun, invented by John T. Thompson in 1919, that became infamous during the Prohibition era. It was a common sight in the media of the time, being used by both law enforcement officers and criminals...

 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

MG 15

The MG 15 was a German 7.9 mm machine gun designed specifically as a hand manipulated defensive gun for combat aircraft during the early 1930s. By 1941 it was replaced by other types and found new uses with ground troops.- History :...

, and the modern Beta C-Mag

Beta C-Mag

The Beta C-Mag is a 100-round capacity magazine designed by Jim Sullivan and adapted for use in numerous firearms firing the 5.56×45mm NATO, 7.62×51mm NATO, and 9×19mm Parabellum cartridges. C-Mag is short for century magazine, referring to its hundred-round capacity. It has two drum units, each of...

. These systems have the advantage of storing even more rounds than a regular drum, while improving the distribution of weight.

The drums of aircraft cannons such as the M61 Vulcan

M61 Vulcan

The M61 Vulcan is a hydraulically or pneumatically driven, six-barreled, air-cooled, electrically fired Gatling-style rotary cannon which fires 20 mm rounds at an extremely high rate. The M61 and its derivatives have been the principal cannon armament of United States military fixed-wing aircraft...

 and GAU-8 Avenger

GAU-8 Avenger

The General Electric GAU-8/A Avenger is a 30 mm, hydraulically-driven seven-barrel Gatling-type rotary cannon that is mounted on the United States Air Force's Fairchild Republic A-10 Thunderbolt II. It is among the largest, heaviest and most powerful aircraft cannons in the United States military...

 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 a helical auger. This makes their operation very reliable, even when operated at rates of fire of several thousand rounds per minute.

The most compact of designs released in the early 1990's was the coil magazine, this particular magazine works with non-protruding rimmed ammunition and is currently manufactured for .308 variants like the M14

M14 rifle

The M14 rifle, formally the United States Rifle, 7.62 mm, M14, is an American selective fire automatic rifle firing 7.62x51mm NATO  ammunition. It was the standard issue U.S. rifle from 1959 to 1970. The M14 was used for U.S...

/M1A1, SR-25

SR-25

The SR-25 is a semi-automatic sniper rifle designed by Eugene Stoner and manufactured by Knight's Armament Company. The SR-25 uses a rotating bolt and a direct impingement gas system. It is loosely based on Stoner's AR-10, rebuilt in its original 7.62×51mm NATO caliber...

, FN-FAL and HK-91. The design allows for magazine to stack against each other with no sperating walls and uses the geometry of the housing to keep the ammunition pressed against each other in a single stack formation.