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 cartridges 9×19mm Parabellum Colt 9mm SMG Heckler & Koch MP5 Uzi Glock, plus these firearms using a Glock magazine: KRISS Vector Gen II Kel-Tec SUB-2000 CZ Scorpion Evo 3 5.56×45mm NATO (STANAG magazine with Mini-14 compatibility) FN Minimi M249 light machine gun Ultimax 100 AR-15 Beretta ARX-160 FN SCAR Heckler & Koch G36 Heckler & Koch HK33 Heckler &