METAL GEAR was the designation given to various models of bipedal walkertanks. They were behemoth machines capable of decimating lesser machines. Metal Gear had the distinction of being nuclear capable, firing warheads from their railgun, eliminating the need for a missile, or launchpad allowing them to attack targets from anywhere in the world, on any kind of terrain. This peaked with Metal Gear REX which was sophisticated enough to launch a nuke and hit a target from anywhere in the world. REX also had the distinction of being completely untraceable.
