Beta C - Mag The Beta C - Mag is a 100 - round capacity magazine manufactured by the Beta Company . It was designed by Jim Sullivan 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 . 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 . 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 7 . 62×51mm NATO Performance evaluations . Reliability . 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 . 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 ) . 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 1000 rounds per minute . Civilian ownership . 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 . However , some states have enacted their own laws against " high capacity " magazines . The C - Mag has been used in a number of high - profile crimes , including the North Hollywood shootout and the 2012 Aurora shooting .