A detachable magazine for storing and delivering ammunition to a firearm having a magazine well. The magazine has a body with a column adapted for insertion into the magazine well. The column has a passage for transmitting ammunition to the firearm. The body includes a drum housing defining a substantially cylindrical chamber communicating with the column passage. A first sprocket element is rotatably received in the chamber and has a serrated periphery, with each serration adapted to receive an ammunition cartridge. A second sprocket element is rotatably received in the chamber, and is concentric with the first sprocket element, having a serrated periphery with each serration adapted to receive an ammunition cartridge. A spring element is connected to at least one of the sprocket elements, and operates to rotatably bias the sprocket elements to transmit cartridges from the drum chamber to the column passage. The magazine may have a pair of drums, and the sprockets may be tapered, to facilitate feeding of tapered cartridges. The magazine may include a counter to indicate the quantity of ammunition consumed or remaining. The magazine may include the ability to power and de-power spring motor.