The Tokyo Marui NGRS M4/SCAR Next Gen Rifle Magazine (430 Rounds – Dark Earth/FDE) is widely available from airsoft retailers. It provides high capacity firepower for compatible Next Gen Recoil Shock rifles, though it disables the "stop on empty" function. 
 
Key Information
Compatibility: This magazine is specifically designed for Tokyo Marui Next Gen Recoil Shock (NGRS) models, including the M4 SOPMOD, SCAR-L, 416, and CQC. It is not compatible with standard M4 AEGs.
Capacity: It is a high-capacity magazine holding approximately 430 rounds of 6mm BBs.
Mechanism: It features a classic high-cap winding wheel mechanism for feeding BBs.
Bolt-Lock Function: The high-capacity nature of this magazine means that the electric blowback rifle's "stop on empty" or bolt-lock function is disabled when using it.
Material and Color: The magazine features a realistic-looking alloy (metal) outer shell with polymer internals and a Dark Earth (FDE/Tan) finish.