One Heck Of A Mag Pouch

Actually, it isn't a mag pouch. It is a backpack that integrates with a machine gun so that the machine gunner can carry up to 925 rounds of 5.56x45 or 575 rounds of 7.62x51 ammunition.

I seem to remember reading a story somewhere that some soldiers in either Afghanistan or Iraq made a prototype using scavenged parts and an old Alice pack frame.