to deal with T-54/55 tanks. The US Marine Corps assigned one M103 company to each of its three Marine tank battalions, including its Marine reserve units. The M103 was never used in combat. While the US Army deactivated its heavy armor units with the reception of the new M60 series main battle tanks in 1963, the remaining M103s stayed within the US Marine Corps inventory until they began receiving the M60 series main battle tank. With the disappearance of the heavy tank from US forces came the full acceptance of the main battle tank in 1960 for the US Army, and 1973 for the US Marine Corps. Although the later M1 Abrams main battle tank utilizes the same caliber of main gun, 120 mm, the M103's cannon was a rifled gun firing a separate-loading round, in which the projectile was loaded into the breech, followed by a cartridge case consisting of a brass case, primer, and propellant in a fixed unit. This separate-loading system necessitated the use of two loaders. The only part of the cartridge case consumed during firing was the propellant and a plastic cap on the end of the brass cartridge case. The spent brass cartridge case was ejected after firing. The M1 tank's 120 mm main gun is a smooth bore firing a semi-caseless round, ejecting only a back cap of the original loaded round; the bulk of the M1's 120 mm shell casing is consumed during firing. Ammunition Ammunition for M103's M58 gun included: M358 APBC-T M356 HE-T M357 WP-T M359E2 TP-T M469 HEAT-T Variants T43 – Six pilot vehicles produced in 1951. T43E1 – 300 built in 1953. T43E2 – Two vehicles produced from 1955–56. Turret basket and gunner moved to front of turret. New targeting system (T52 rangefinder, T33 computer, T44 gunners periscopic sight) and hydraulic turret traverse replaced with electric M103 – Produced in 1957. 74 converted into other models. M103A1 Produced in 1959. 219 converted or rebuilt. New sight (Stereoscopic T52) and M14 ballistic computer. Removed one coaxial machine gun. New turret electric amplidyne system traverse. Turret basket. M103A2 Produced 1964. 153 converted or rebuilt. New 750 hp (559 kW) diesel engine from the M60 tank, increasing the road range to 295 mi (480 km) and maximum speed to 23 mph (37 km/h). The M15 Stereoscopic Rangefinder is replaced to M24 Coincidence Rangefinder. Heavy Recovery Vehicle M51 Initially built 1954–1955 and