C-Mag

From Airsoft Community Wiki

A C-Mag is a type of high capacity magazine used with both Real steel weapons (produced by BetaCo) and Airsoft replicas. It consists of two drums either side of the magazine well, and in real weapons hold 100 rounds (hence C-Mag for "Century Magazine") and feed in the same manner and a standard magazine where in airsoft it could contain several thousand and must be wound either electrically or manually to feed.

They are a more realistic alternative to generic box magazines in airsoft, and are available for almost every type of type of airsoft rifle as well as the MP5 series.