Umarex Steel Strike Drop-Out Magazine
30-round magazine with 900-round resevoir 
For use with Umarex Steel Strike CO2 Air Rifle 
Integrated pull-tab transfers BBs from reservoir to magazine on or off rifle

 
Grab a spare magazine or two for your Umarex Steel Strike air rifle and you could conceivably shoot for hours on end without having to refill the integrated reservoir that keeps up to 900 BBs at the ready inside the mag.