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.[1] C-Mag is short for century magazine, referring to its hundred-round capacity. It has two drum units, each of which hold half of the cartridges inserted into the magazine. The latest version of the magazine is available with a transparent backing to allow the user to see the number of rounds remaining in the magazine.[2] A C-Mag typically weighs about 2.1 kg (4.63 lb) when loaded with 5.56×45mm NATO ammunition.
One such magazine has been designed for the M16 rifle, in use by the US Military. The NATO Stock Number for the M16 version is 1005-01-363-6670. The magazine design, including drawings, is covered in detail in U.S. Patent 4,658,700.
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The C-MAG is a compact twin-drum magazine design that accepts up to 100 rounds of ammunition. It consists of two main components: the twin-drum storage housing, and an interchangeable feed clip assembly. The storage housing is standard and fits any like-caliber weapon. The feed clip assembly serves as an adapter for the specific weapon.
Before loading and after firing, the feed clip is filled with spacer rounds that are an integral part of the magazine. The upper half of the top spacer round is tapered to allow the weapon bolt to close after the last round is fired. The length of the string-set depends on the customized feed clip for the individual weapon.
During loading, cartridges are inserted on top of the spacer rounds, through the feed clip and into the drums. The cartridge column splits at the juncture of the feed clip and the housing to distribute the ammunition evenly into the drums in two concentric rows.
During firing, spring-driven rotors advance the cartridges in both drums until they meet at a cam blade that merges the cartridges into a single column that feeds up through the feed clip and into the weapon.[3]
A test in 2003 by US Army soldiers in Afghanistan found the C-Mag unreliable in simulated combat conditions, with frequent failures to feed among the issues.[4] Problems are exacerbated in dusty and sandy environments. The C-Mag also stresses the magazine release, since a fully loaded C-Mag weighs more than three fully loaded 30 round magazines.[5] U.S. Army TACOM has issued a Ground Precautionary message, GPM-02-017, warning about its use in operations.[6] The GPM predates the initiation of Operation Iraqi Freedom. The Beta C-Mag is not in widespread use by US military forces, and has not been type-classified.
In November 2008 the Army Experimental Task Force (AETF) at Ft. Bliss, TX, evaluated six BETA C-MAG magazines. Four magazines—two with black covers and two with clear covers—were used with M4 carbines in three firing scenarios: controlled pair, controlled burst, and rapid fire. According to the memorandum summarizing the evaluation, the four magazines “performed flawlessly in all three scenarios without jams or stoppages.” In addition, two magazines with black covers were evaluated with M249 light machine guns in controlled burst and rapid-fire scenarios. These also performed without “issues,” according to the memo, which also notes that soldiers “had only positive comments” about the C-MAG magazines during the After Action Review (AAR).[7]
The interchangeable feed clip (or "tower") on older Beta C-Mag editions have some problems in later AR-type weapons with closer tolerances for the magazine well. The old feed clip which inserts into the rifle's magazine well is encircled by "ribs" molded into the exterior of the upright tower. As a result, the magazine cannot be inserted into the magazine well far enough to fully seat.
In 1994, the United States Federal Assault Weapons Ban passed, prohibiting manufacture of magazines with capacity of more than 10 rounds for civilian use. Civilian purchase and ownership of previously manufactured magazines was legal, though as a result of a limited supply, the C-Mags became rare and expensive. The ban expired on September 13, 2004, and Beta Company has resumed production of the C-Mag for the civilian market.[2] Some states though, have enacted their own laws against "high capacity" magazines.