Royal Ordnance L11A5 The Royal Ordnance L11A5 is a 120 mm L / 55 rifled tank gun design . It was the first of NATO ' s 120mm tank guns which became the standard calibre for Western tanks in the later period of the Cold War . A total of 3 , 012 L11 guns were produced at a reported unit cost of US $ 227 , 000 . The L11 was developed by Britain ' s Royal Ordnance Factories to equip the Chieftain tank as the successor to the 105 mm L7 gun used in the Centurion tank . It was also used on the Challenger 1 , which replaced the Chieftain in British and Jordanian service . The weapon has been superseded by the L30 series 120 mm rifled tank gun . History . The Royal Armament Research and Development Establishment at Fort Halstead designed a new 120mm rifled tank gun in 1957 . The new gun was deemed to be necessary because the British Army specified engagement range s greater than those of other armies , for example , as specified by the US Army , despite studies at the time that suggested engagement ranges were below those of the US Army requirements in the great majority of cases . The L11 was specifically designed to fit into the turret mountings of the Chieftain tank ( FV4201 ) . After firing trials in 1961 , the L11 was accepted for service on the Chieftain in 1965 , and entered service with the British Army in 1966 . The British Army decision to adopt a rifled gun as opposed to a smooth - bore which other NATO nations favoured was : Since its introduction , the L11 has evolved into eight production versions . In June 1976 development of new ammunition for the L11A5 was begun . Production Models . Royal Ordnance basic L11 design was developed into a series of improved production models ; the : L11A5 being the major production version . Technology . The breech mechanism is a downward sliding semi - automatic breechblock . The gun was equipped with a hydro - pneumatic recoil system using two buffers . The gun recoil s in most applications . Unlike most tank weapons which fire a single fixed round , the round ( projectile ) and propellant are loaded separately . The propellant is in the form of a combustible " bag " charge ( or later , a combustible charge case for armour - piercing rounds ) . This required the obturation to be provided by the breech rather than the cartridge case , as is the case in fixed rounds . When first introduced , APDS ( armour - piercing discarding sabot ) rounds were fired using a cylindrical charge . High explosive squash head ( HESH ) , smoke and other rounds used a hemi - cylindrical ( i . e . a cylinder sliced in two lengthways ) charge ( the L3 ) . Two HE charges could therefore be stowed in the same space as one AP charge . In the Chieftain and Challenger tanks , the charges were stored in 36 recesses surrounded by water jackets , so that a hit which penetrated the fighting compartment would rupture the jacket and drench the propellant , preventing a catastrophic ammunition fire ( known colloquially as a " brew - up " ) . The barrel of the L11A5 is fitted with a bore evacuator approximately two - thirds of the way to the muzzle and a thermal sleeve . When first introduced , a 12 . 7 mm ( . 50 inch ) in calibre ranging gun was fitted over the barrel of the L11 . The projectiles for this ballistically matched those for the main armament out to , at which point the tracer element burned out . This effectively limited the maximum range for the main gun to this distance . In the late 1970s , laser rangefinders replaced the ranging MG in British service , allowing engagements at longer ranges . However despite this , testing at the US Army Aberdeen Proving Ground concluded that engaging targets beyond is not practical due to round deviation . This is especially true against targets that are moving . During Operation Granby an L11 on a British Army Challenger 1 scored the longest tank - to - tank ' kill ' in military history , when it destroyed an Iraqi T - 72 at a range of 5 . 1 km ( 3 . 1 mi ) .