mm main gun, had a hull with a straight front slope whereas the M48's hull was rounded, had three support rollers per side to the M48's five, and had road wheels constructed from aluminum rather than steel. The hull of the M60 was a single piece steel casting divided into three compartments, with the driver in front, the fighting compartment in the middle, and the engine at the rear. The driver looked through three M27 day periscopes, one of which could be replaced by a night vision periscope. Initially, the M60 had essentially the same turret shape as the M48, but this was subsequently replaced with a distinctive "needlenose" design that minimized frontal cross-section to enemy fire. The M60 was the last U.S. main battle tank to utilize homogeneous steel armor for protection. It was also the last to feature either the M60 machine gun or an escape hatch under the hull. Originally designated the M68, the new vehicle was put into production in 1959, reclassified as the M60, and entered service in 1960. Over 15,000 M60s (across all variants) were constructed. M60A1 In 1963, the M60 was upgraded to the M60A1. This new variant, which stayed in production until 1980, featured a larger, better-shaped turret and improvements to the armor protection and shock absorbers. The M60A1 was also equipped with a stabilization system for the main gun. However, the M60A1 was still not able to fire on the move, as the system only kept the gun pointed in the same general direction while the tank was traveling cross country. It did however enable the coaxial machine gun to be brought to bear while moving. Post-Vietnam Light tanks M551 Sheridan The Army reclassified their armor (tanks) in 1950 from heavy, medium, and light tanks to 120mm, 90mm and 76mm gun tanks, etc. Consequently, when the M551 Sheridan entered combat in Vietnam in January 1969, it was assigned to strictly cavalry units, and was classified as an Armored Reconnaissance Airborne Assault Vehicle M551 and not a light tank. The last official U.S. light tank in service was the M41 Walker Bulldog, and that vehicle was never used in combat by U.S. forces; being transferred to Army South Vietnamese forces (ARVN) and U.S. Army Guard/Reserve forces until being phased out of general service, excepting special service units: Airborne, OPFOR (Opposing Force-Training), Ft. Irwin, etc. Airborne units began to phase out the Sheridan