 Because of concerns about the inability of the 105 @-@ millimetre ( 4 @.@ 1 in ) L7 tank gun then in use across NATO forces to penetrate new Soviet armor , as proved in German tests on four T @-@ 62 Soviet tanks captured by Israel following the June 1967 Six Day War , Rheinmetall was paid for the development of a new tank gun , a project started in 1965 , as the Bundeswehr felt a more powerful gun was needed for its new tanks . The first instance of a larger Soviet tank gun was witnessed on the chassis of a modified T @-@ 55 in 1961 . In 1965 , the Soviet Union 's T @-@ 62 made its first public appearance , armed with a 115 @-@ millimeter ( 4 @.@ 5 in ) smoothbore tank gun . The Soviet decision to increase the power of its tank 's main armament had come when , in the early 1960s , an Iranian tank commander defected over the Soviet border in a brand @-@ new M60 Patton tank , which was armed with the British Royal Ordnance L7 . Despite the introduction of the T @-@ 62 , in 1969 their T @-@ 64 tank was rearmed with a new 125 @-@ millimeter ( 4 @.@ 9 in ) tank gun , while in 1972 Nizhny <unk> began production of the T @-@ 72 tank , also armed with the 125 @-@ millimeter ( 4 @.@ 9 in ) gun . For example , at the fighting at Sultan <unk> , during the 1982 Lebanon War , the Israeli government claimed to have destroyed nine Syrian T @-@ 72s with the Merkava main battle tank , armed with an Israeli production version of the American M68 105 @-@ millimeter ( 4 @.@ 1 in ) tank gun ( which in turn was based on the British L7 ) . Whether or not true , the Soviets test @-@ fired a number of Israeli <unk> Hetz armor @-@ piercing discarding sabot rounds at Kubinka , finding the 105 @-@ millimeter ( 4 @.@ 1 in ) round was able to perforate the T @-@ 72 's sloped front section plate but not its turret armor . In response , the Soviets developed the T @-@ <unk> . This led Israel to opt for a 120 mm tank gun during the development process of the Merkava III main battle tank . This case is similar to the American decision to replace the M68 105 @-@ millimeter ( 4 @.@ 1 in ) tank gun with Rheinmetall 's 120 mm gun in 1976 ; the introduction of the T @-@ 64A had raised the question within the armor community whether the new ammunition for the existing gun caliber could effectively deal with the new Soviet tank . 
