Royal Ordnance L11
The Royal Ordnance L11A5, officially designated Gun 120 mm Tk L11, is a 120 mm L/55 rifled tank gun design. It was the first of NATO's 120mm main battle tank guns which became the standard calibre for Western tanks in the later period of the Cold War. By 2005, a total of 3,012 L11 guns were produced. List price was US $227,000 (1990). 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 The weapon has been superseded by the L30 series 120 mm rifled tank gun.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 120 mm rifled tank gun in 1957. The new gun was deemed to be necessary because the British Army specified engagement ranges 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. 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-55 at a range of 4.7 km (2.9 miles) with an L23 "Fin" round. 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. The Royal Ordnance basic L11 design was developed into a series of improved production models; the L11A5 was the major production version. Design. 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 recoils in most applications. This breechblock design was based on the breechblock on the Krupp/Skoda sFH 18/43 model 18 that the British studied extensively after the Second World War and perfected for use in the 120 mm gun. Unlike most tank weapons which  fire a single fixed round, the 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 in) calibre ranging gun was fitted over the barrel of the L11. The projectiles for this  ballistically matched those for HESH rounds fired from the main armament out to , at which point the tracer element burned out. Starting in 1971 a Barr & Stroud LF2 "Tank Laser Sight" (TLS) laser rangefinder replaced the ranging MG in British service, and in conjunction with the "Muzzle Reference System" (MRF) added in 1975, allowed engagements at ranges out to 5,000 meters. Further improvement in gunnery performance came with the adoption of the Marconi "Improved Fire Control System" (IFCS) fitted to the Chieftain in 1979.