This is an auto winding drum magazine for A&K M249 SAW variants. The capacity is approximately 2,500 rounds. The BBs are fed to the gun using an electric feeding mechanism that is battery powered. This magazine is compatible with all A&K M249 SAW variants, including the PARA, MK I, and MK II . It is also compatible with some other brands of the M249. We do not guarantee comparability with any brand other than A&K. This magazine has three settings: auto, sound control, and off. The "auto" setting winds the magazine until it is turned off. You will hear a clicking noise when the magazine is fully wound. At that point it should be turned off. The "sound control" feature causes the magazine to automatically feed BBs when it detects the sound of the gun firing. The "off" setting turns the magazine off.