One Full-Metal Hi-Cap M4 Magazine not enough? - see NUPROLs electronicly controlled double magazine assemblies.
Two Hi-Cap metal magazines joined together to provide a whopping 800R / 1200R capacity. Features an electronic tensioner - merely hold the button toward the front of the magazine to keep the BBs coming. Alternatively, purchase the "SC" (Sound Controlled) version that maintains tension as the shots are fired. For when batteries run-flat, a manual tensioner wheel is provided on the base.
Requires (x4) AAA