Beta C-Mag

The Beta C-Mag is a 100-round capacity drum magazine manufactured by the Beta Company. It was designed by Jim Sullivan and first patented in 1987 and has been 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 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. A C-Mag loaded with 5.56×45mm NATO ammunition typically weighs about ; a C-Mag loaded with 7.62×51mm NATO ammunition weighs .
A version adapted for the M16 rifle is used by the U.S. military. The magazine design, including drawings, is covered in detail in .
Design.
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.
Firearms compatible with C-MAG magazines.
9×19mm Parabellum
5.56×45mm NATO (STANAG magazine with Mini-14 compatibility)
7.62×51mm NATO
Performance evaluations.
Reliability.
A test in 2003 by U.S. Army soldiers in Afghanistan found the C-Mag unreliable in simulated combat conditions and reported frequent failures to feed among the issues; likewise, British Armed Forces trials found that the C-Mag did not give reliable performance when loaded with British-issue ammunition. The Beta C-Mag is not in widespread use by U.S. military forces, and has not been type-classified.
In November 2008, the U.S. Army Experimental Task Force (AETF) at Fort Bliss, Texas, 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." Additionally, 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).
Test-fired with a Heckler & Koch HK91, Heckler & Koch G3 and two other Heckler & Koch-based weapons, the 7.62mm C-Mag was found to operate without problems in either loading or firing at rates up to 1,000 rounds per minute.