Royal Ordnance L7

During the Hungarian Revolution of 1956, a Soviet T-54A medium tank was driven onto the grounds of the British embassy in Budapest by the Hungarians. After a brief examination of this tank's armor and 100 mm gun, British officials decided that the 20 pounder was apparently incapable of defeating its frontal armor. This meant the most common British tanks were no longer able to deal with Soviet medium tank designs, let alone their heavy tanks.

These events spurred the United Kingdom to develop a new tank gun in 1956, the Royal Ordnance L7 to keep the Centurion viable against this new Soviet tank design and the United States to develop the XM60 tank in 1957. The L7 was specifically designed to fit into the turret mountings of the 20 pounder. This would enable the Centurion tanks to be up-gunned with minimum modifications; hence, the fleet could be upgraded in a shorter time and at a lower cost.

User trials of the weapon began in 1959. The first tank to be equipped with the L7 was a single up-armoured Centurion Mark 7 in 1959 which was to prove the viability of up-armouring and up-gunning the Centurion. From 1959 onwards existing Centurions were given upgrades with the L7 gun and armour and new builds incorporated the L7 at production.[3]

The main gun for the M60 tank series was chosen after a comparative firing test of six different guns carried out on the Aberdeen Proving Grounds in 1958. The factors evaluated were accuracy, the lethality of a hit, rate of fire, and penetration performance.[4] Based on these tests, the 105 mm T254E1 was selected, modified to the T254E2 and standardized as the Cannon, 105-MM Gun, M68. They are licensed built US variants of the L7. The T254E2/M68 used a vertical sliding breechblock instead of the T254E1's horizontal breechblock. Until American-made barrels could be obtained with comparable accuracy, British X15/L52 barrels mounting a concentric bore evacuator on the barrel were to be used.[5] US built XM24/L52 barrels (length 218.5 inches)[6] fitted with an eccentric bore evacuator were used for the M60-series starting in June 1959 but retained interchangeability with the British X15/L52 barrel.[7] All of the US guns and XM24 barrels were produced at the Watervliet Arsenal, NY and the gun mounts (M116 for the M60 and M140 for the M60A1/A3) manufactured at the Rock Island Arsenal, IL.[8] US M68 guns were fitted with an eccentric bore evacuator instead of a concentric model in order to provide more clearance over the rear deck of the tank.[9] The original variant of the M60 tank was equipped with the M68 gun using the M116 mount.[10] Additionally, many M48A3s armed with a 90mm gun that were in NG-CONUS service with the Army National Guard were retrofitted with the M68 gun and redesignated as the M48A5.[11] This was done to maintain training levels of Guard units as well as using a commonality in ammunition amongst tanks.

The M60A1 and A3 variants of the M60 series[12] and earliest pre-production XM1 prototypes of the M1 Abrams tanks[13] are armed with the M68E1 variant of the gun. The M68E1 gun shares the same firing characteristics as the M68. It featured several design improvements including an updated gun hydraulic configuration, a stabilization upgrade for the gun, a gun elevation kill switch for the loader, improved ballistic drive and other component refinements.[14] They were fitted with thermal sleeves on the barrels starting in 1973.[15] During the mid 1970s it was becoming clear that the latest generation of composite based armor was impervious to tungsten carbide penetrators.[16][17] Work was performed at the Department of Energy's Pacific Northwest National Laboratory to engineer development of depleted uranium as a penetrator material for future ammunition while the Armament Research and Development Command (ARRADCOM) was improving the performance of the 105mm M774 cartridge.[17][18] The M744A1 cartridge was issued starting in 1980 using a depleted uranium penetrator to keep the M68E1 gun viable against this improved armor. In 1975 an updated version of the gun, the T254E3 was designed, focusing on the use of chrome plating to improve accuracy. It was used to evaluate improvements to the gun's performance using discarding sabot ammunition. Two guns were built and underwent firing trials at Aberdeen[19] and technical evaluations at the Watervliet Arsenal.[20] Based on the results of these tests the shortcomings of plated bores and gun tubes were found to outweigh any advantage they might offer and the program dropped by May 1976.[21]

In January 1978, a program was initiated[22] to develop an enhanced version of the 105mm gun, the M68A1 as a possible alternate weapon for the M1 Abrams.[23] The new XM24/L55 gun barrel was 18 inches (45.72cm) longer in comparison to the XM24/L52 barrel used on the M60-series tank.[24] It has a higher chamber pressure,[25] reinforced breach[23] and a higher muzzle velocity.[22][26] Although the M256 120mm smoothbore gun was chosen to be the main weapon of the M1 Abrams in 1979, the ammunition for the gun was still not fully developed, thus delaying its fielding until 1984.[16] The early production versions of the M1 Abrams (M1 & IPM1) were armed with the M68A1[27] for two reasons. First was due to the large number of M60 Patton tanks with the M68E1 gun still in widespread US service in the 1980s and a large on-hand stockpile of 105mm munitions. Fitting the M1 with the M68A1 gun was viewed as an economical and practical solution that allowed for commonality in ammunition among the two types of tanks.[28] Secondly was the fact that the M68A1E2 could employ the newly developed M900 APFSDS[29] depleted uranium round that had improved penetration performance in comparison to the M774.[30] These early versions of the M1 Abrams were in active Army service until 1991 and with National Guard units until 1996.[31] M1s built after 1984 were armed with the 120mm M256 and designated the M1A1. Many earlier M1 and IPM1 tanks were refitted with the M256 and their designations changed to the M1A1.

The M1128 Stryker Mobile Gun System (MGS) carries the M68A1E4 105mm cannon. The principal function of the MGS is to provide rapid direct fire to support assaulting infantry.[32] The cannon is mounted in a low-profile, fully stabilized turret integrated into the Stryker chassis.[33] The M68A1E4 is based on the M68A1E2 105mm cannon's design. It has a sustained fire rate of six rounds per minute. The gun employs four types of cartridges. The M900 kinetic energy penetrator to destroy armored vehicles; the M456A2 high explosive anti-tank round to destroy thin-skinned vehicles and provide anti-personnel fragmentation; the M393A3 high explosive plastic round to destroy bunkers, machine gun and sniper positions, and breach openings in walls for infantry to access; and M1040 canister shot for use against dismounted infantry in the open.[34] As of 2015, the M68A1E4 is the only variant of the M68 105mm gun still in active service with the United States.

The gun was subsequently adopted for use on the German Leopard 1 (for which the L7A3 variant was developed) and the Swiss Panzer 61 and Panzer 68. The L7A3 was also adopted by Japan for the Type 74 tank. The M68 gun is also featured on the Israeli Magach 6 and 7 series as well as early versions of the Merkava MBT. The Swedish Stridsvagn 103 turretless S-tank (armed with the longer Bofors L74 gun with an automatic loader) makes use of an indigenous gun design, which is compatible with ammunition made for the L7. In addition, several countries have used the gun to improve the firepower of existing main battle tanks. Derivatives have even been mounted in Warsaw Pact-built T-54 and T-55 tanks in Israel, India, Egypt and Iraq, and Type 79 tanks in China.

• L7A1

Standard UK production variant.

• L7A2

L7A1 fitted with a thermal sleeve.[36]

• L7A3

Variant for the (West) German Leopard 1 MBT. The upper rear corner of the breech block reduced in size so gun can be depressed without hitting the turret roof.

• L7 LRF

Low Recoil Force (LRF) version of the L7 used by the Stingray light tank.

• Type 79/81/81A/83

Chinese produced L7. Licence procured from Austria.

• FM K.4 Modelo 1L

Argentine Army's licence produced by Fabricaciones Militares in Argentina. Used on the TAM medium tank.

• T254E1

US designation for the L7A1. It had the same horizontal sliding breach block as the L7 and used British X15/L52 tubes with a concentric bore evacuator on the barrel.

• T254E2

United States variant of the T254. It had a vertical sliding breach block with the X15E8 tube and a concentric bore evacuator. Standardized as the M68. Used in M60 Patton prototype vehicles.

• T254E3

US variant designed in 1975. Same as the T254E2/M68 but with chrome plated bores. Only two built. [37][38]

• M68

Initial production variant. Used on the original variant of the M60 tank and the M116 gun mount.[39] Retrofitted to the M48A5.

• M68E1

US variant used on the M60A1 and M60A3 tanks in mount M140. They were fitted with a fiberglass thermal shroud[40] in 1973.[15] It was also used as a place holder on early XM1 developmental prototypes while the preferred gun, the L11/M256, was in development.

• M68A1

US variant built in 1980[41] for use on the M1 and IPM1 versions of the M1 Abrams MBT[42][43] fitted with a 1.5 ft (460 mm) longer (total length 236.5 inches) XM24E4/L55 gun tube and re-enforced breach[44] for use with M900 depleted uranium cartridge.[29] The guns were fitted with an aluminium thermal shroud and a muzzle reference sensor on the barrel.[45]

M68A1E4 autoloader System

• M68A1E4

Variant of the US M68A1 designed for use on the M1128 Mobile Gun System (Stryker MGS), it features an Ares Incorporated long stroke low impulse recoil mechanism and a modified M68A1 breech.[46]

• KM68A1

licence-produced variant of the US M68A1 gun for the South Korean Army. Used on the South Korean variant of the M48, the M48A5K and K1 Type 88 tanks.

• M68T

Turkish licence-built versions by MKEK under the designation of M68T to up-gun its 90 mm armed M48 fleet, took place in the 1980s.

• M64 L71A

Israeli variant built by IMI for the Merkava Mk. I and Mk. 2 tanks.

• GT-7

South African variant built by Denel for the Olifant Mk1A and Rooikat 105. Incorporates a heavily modified recoil assembly.

• L74

Swedish model built by Bofors for the Strv 103 using more sophisticated recoil assembly and a longer L/62 gun tube for increased muzzle-velocity and accuracy.

• Centurion: L7A1 cannon on late Centurion models and derivatives such as Olifant
• Vickers MBT
  • Vijayanta
• Leopard 1: L7A3 cannon
• Type 74: L7A1 with M68 breechblock
• M48 Patton: in some upgraded variants
  • M48A2GA2: West German modification
  • M48A5E: Spanish modification
  • M48A5T2: Turkish modification
• T-54/55: in some upgraded variants
  • Type 59-II: Chinese modification of T-54
  • Tiran-4Sh: Israeli modification of T-54, both L7 and M68 variants fitted
  • Tiran-5Sh: Israeli modification of T-55, both L7 and M68 variants fitted
• Pz-61
• Pz-68
• TAM medium tank
• Type 05: Chinese amphibious light tank
• Type 62G: Chinese light tank
• Type 63A: Chinese amphibious light tank
• Type 79: Chinese MBT
• Type 69-IIG: An upgraded Bangladeshi variant of Type 69 tank
• Type 88: Chinese MBT

• M1 Abrams: M68A1E2 cannon used early production models (M1 and IPM1)
• M47 Patton: in some upgraded variants
  • M47M: Iranian modification of US-supplied M47Ms
  • Sabalan: Iranian modification
  • Tiam: Iranian modification
• M48 Patton: in some upgraded variants
  • M48A5: US model
  • Magach 3: Israeli modification
  • CM-11 Brave Tiger: Taiwanese modification
  • M48A5K1: South Korean modification
  • M48A5T1: Turkish modification
• M60 Tank[47]
• M1128 Mobile Gun System: M68A1E4 cannon

• K1 Type 88: KM68A1 cannon
• Merkava: Mark I and Mark II models
• T-54/55: in some upgraded variants
  • Tiran-4Sh: Israeli modification of T-54, both L7 and M68 variants fitted
  • Tiran-5Sh: Israeli modification of T-55, both L7 and M68 variants fitted
  • Ramses II: Egyptian T-55 modernization.

• EE-T1 Osório
• OF-40
• Stingray light tank
• Strv 103
• M47 Patton: in some upgraded variants
  • M47E1: Spanish modification armed with a Rheinmetall RH-105/30 gun
  • M47E2: Spanish modification armed with a Rheinmetall RH-105/30 gun
• Type 16 Maneuver Combat Vehicle: Japanese wheeled armored vehicle with Japan Steel Works 105mm/52caliber gun.