Metal Gear REX was a Metal Gear prototype that was jointly developed by DARPA, ArmsTech, and Livermore National Labs for the United States Army in the early 2000s. REX differed from earlier Metal Gear models in that its legs were heavily armored and reinforced; not vulnerable like its predecessors. Along with near-impenetrable compound armor (of which only HEAT weapons could deal sufficient damage), REX also had a pair of 30mm, XGAU-8R rotary cannons, AGM-114P anti-tank missiles,and a free-electron variant of the Mobile Tactical High-Energy Laser (MTHEL-FE) to protect itself from conventional forces. Its feet also possessed retractable metal stake-like protrusions, which were used to find purchase on difficult terrain, and could be employed in "stomp" attacks on an enemy. The AGM-114P anti-tank missiles, launched from both of REX's knees and back, were a laser semi-active homing type that didn't use wires. The launchers used a low-powered laser illuminator to bounce a coded laser beam off a target that the missile would then home in on. The free-electron laser cannon mounted on REX's belly was capable of generating almost 100 megawatts of energy, ten times greater than any other laser at that time. REX's most fearsome weapon, however, was the magnetic rail gun capable of delivering an untraceable nuclear warhead anywhere in the world, without the propellant trail or launch flare that gives away the launch position of a traditional ballistic missile. By perfecting the process of electromagnetic acceleration, the rail gun is able to fire a projectile with a muzzle velocity of over 100 kilometers per second. It was apparently originally going to be used, as part of SDI, to shoot down enemy ICBMs outside the atmosphere. Otacon stated that when fired outside the atmosphere, REX's nuclear rounds would have a radius of effect of 300 meters, and the gun would be sufficiently accurate to hit a satellite traveling at 10 kilometers per second. The rail gun's feature was also implied by ArmsTech President Kenneth Baker to be illegal, and that were it to be discovered, his company would most likely end up shut down and Baker himself would most likely face an inquiry.
