Drum magazine A drum magazine is a type of firearms 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 . Small arms . 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 . 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 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 . Heavy weapons . 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 .