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 The weapon has been superseded by the L30 series 120 mm rifled tank gun.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 ranges greater than those of other armies, for example 2000 m, 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:
 * 1) Rifled tank guns can (in addition to firing conventional spin-stabilized ammunition) fire fin stabilized ammunition by using a slipping driving band. This allows employment of the widest range of ammunition.
 * 2) Spin-stabilized ammunition, as fired from rifled tank guns, has inherent advantages of stability in flight.
 * 3) A rifled piece is more economical to procure and maintain.

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.
 * L11A1 – The initial production variant; 130 produced.
 * L11A2 –RO Defence incorporated numerous minor changes, including a modified vent tube, an obturator sleeve protector, and a 15-hole vent tube magazine. Stronger material was used in fabricating the breech ring.
 * L11A3 – This incorporated minor changes to the breech ring.
 * L11A4 –Evaluation test prototype for an automatic loading system.
 * L11A5 – This was the main production model. It introduced the integral muzzle reference system and a smaller and lighter fume extractor which required the addition of 7.7 kg of weight at the breech for balance.
 * L11A6 – This was a conversion of the A3 to accommodate the muzzle reference system and fume extractor of the A5.
 * L11A7 – A semi-automatic plunger was proposed for the vent tube loader, but did not enter production.
 * L30 (EXP 32M1) – The latest variant of the L11 design, developed under the Challenger Armament program.

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 recoils 37 cm 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 2600 m, 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 3 km 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).

Specification

 * Calibre: 120 mm
 * Barrel length 6.604 m (55 calibres)
 * Length overall 6.858 m
 * Weight: 1778 kg
 * Recoil distance: 37 cm
 * Maximum range/velocity (APDS): 3000 m, 4495 ft/s
 * Maximum range/velocity (HESH): 8000 m, 2198 ft/s
 * Maximum rate of fire: 10 rounds/min
 * Sustained rate of fire: 6 rounds/min
 * Elevation: +20/-10 on Chieftain Mk 2.

Available ammunition

 * L31 High Explosive Squash Head (HESH)
 * L15 Armour Piercing Discarding Sabot-Tracer (APDS-T) (production discontinued)
 * L20 Discarding Sabot-Tracer (DS-T)
 * L23 Armour Piercing Fin Stabilized Discarding Sabot (APFSDS)
 * L34 Smoke
 * L32 Squash Head-Practice (SH-P)
 * L35A1 canister shot

Users

 * British Army
 * Iranian Army
 * Jordanian Army