The Beta C-Mag is a drum magazine designed by Jim Sullivan.
As it is classified as a drum magazine, it has a capacity of 100 rounds of either 5.56x45mm NATO, 7.62x51mm NATO, and 9 x 19 mm Parabellum. C-Mag apparently stands for Century Mag. C-Mags can wiegh quite a bit, as when loaded with 5.56x45mm NATO, it weighs 4.62 lbs.
They are known to malfunction frequently leading MAG-SAW, Magpul and other manufacturers to create new magazines with more rounds and better reliability.
They are compatible with the AR-15 rifles, HK G36 rifles, SG 550 rifles (although rare) and MP5 SMGs.