The US Marine Corps assigned one M103 company to each of its 3 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 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 M1 Abrams main battle tank utilizes the same caliber of main gun, 120mm, 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 120mm 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 120mm shell casing is consumed during firing. The Mech’s M103 is 1 of 153 that were upgraded to A2 status in 1964.