Of all of Muscle Tower, Goku's second challenge - to get past Sergeant Metallic (a.k.a. Full Metal Jacket) - was by far the most difficult task. Since Metallic was a robot, he was very hard, and in all truth the only reason Goku won was because the major's batteries ran out of power. At one point in the anime, Goku accidentally blows off his head using a Kamehameha, but even that doesn't stop Metallic from continuing its' rampage. Even without a head and eventually losing both wrists, he tried to repeatedly stomp Goku flat. It was the first time in the series someone was actually able to take a couple hits from the Saiyan warrior and survive. Metallic appeared for a couple seconds in Dragonball GT, firing his fist off and causing major damage to a building. Stylistically, he is an obvious tribute/parody of The Terminator played by Arnold Schwarzenegger.
