Drum Magazine - Small Arms

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 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 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. But the drum magazine also suffered a high-profile failure in July 2012. Movie theater gunman James Holmes had obtained one for his AR-15 rifle but it jammed, limiting casualties to 12 dead and 59 wounded.

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 magazine 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.