 Production of the German Leopard 2 and the new 120 mm tank gun began in 1979 , fulfilling an order for the German Army . Although the American M1 Abrams was originally armed with the <unk> 105 mm gun ( a version of the L7 ) , the United States Army had planned to fit the tank with a larger main gun at a later date , and the tank 's turret had been designed to accommodate a larger 120 mm gun . The larger gun was integrated into the M1A1 Abrams , with the first vehicle coming off the production line in 1985 The gun , known as the M256 , was based on the L / 44 tank gun , although manufactured at Watervliet Arsenal and modified to increase the resistance of the barrels to fracture and fatigue . Tanks armed with versions of Rheinmetall 's gun produced under licence include Japan 's Type 90 and South Korea 's <unk> . The gun had made a huge turn in technological history . 
