https://www.crw-airsoft.com/ghk---40rds-gmag-gas-magazines-for-ghk-m4a1-gbb-black-12043-p.asp
GHK - 40rds GMAG Gas Magazines for GHK M4A1 GBB Black
Spare GMAG gas magazine for the gas blowback M4 series from GHK. Holds up to 40x 6mm BBs and comfortably takes Green gas for normal operation but is
gas magazinesghkgmaggbbblack