Here is the entry in the CSC for the High-capacity Magazine
High-capacity Magazine: The ability for a weapon to hold more ammunition before requiring a change in magazine is always beneficial, especially on high rate-of-fire automatic weapons. High-capacity magazines, while bulky in use, are designed to solve this problem. Multiply the mass and cost of a weapon’s standard magazine to purchase a high-capacity version. The referee may rule that some weapons (such as many shotguns) cannot use high-capacity magazines.
Item TL Kg Cost
High-capacity Magazine 4 x2 x3