 In 1963 , Germany and the United States had already embarked on a joint tank program , known as the MBT @-@ 70 . The new tank carried a three @-@ man crew , with the driver in the turret , an automatic loader for the main gun , a 20 @-@ millimeter ( 0 @.@ 79 in ) autocannon as secondary armament , an active hydropneumatic suspension and spaced armour on the glacis plate and the front turret . The new tank concept also had improved armament , a 152 @-@ millimeter ( 6 @.@ 0 in ) missile @-@ launching main gun , designed to fire the MGM @-@ 51 Shillelagh anti @-@ tank missile . However , the German Army was interested in a tank gun which could fire conventional ammunition . Although there were attempts to modify the 152 @-@ millimeter ( 6 @.@ 0 in ) tank gun to do so , the process proved extremely difficult , and the Germans began development of the future Rheinmetall 120 mm gun instead . 
