This is the Surefire 60 round .223 5.56mm for AR-15, SA80 and STANAG format rifles.
Surefire were pioneered quad stack AR magazines, their patented 4x2 quad stack design the magazines offer a high capacity without the weight and bulk of a drum mag. 60 rounds in the same length as a standard 30 round magazine and still narrower than x2 30 round magazines clipped together.
Can be inserted on a closed bolt. The springs can be left loaded indefinitely. Ideal for LSW use.