Pretty magazine cartridge enveloped by sand-painted nylon. Universal to both TOP and Classic Army M249: two specialized adaptors to dock the TOP's and CA's model respectively. Unzip to load BBs and attach the 9V battery. Remote switch can be hided, so fighting without disturbance!
More photos: The magazine adaptor, Battery connector, Remote switch.
Disclaimer: All aftermarket upgrade parts require necessary skill or modification to install and assemble, please consult with your airsoft technician if needed.