| Ordnance QF 20 pounder Mark I | |
| Origin: | United Kingdom |
| Is Artillery: | yes |
| Is Uk: | yes |
| Is Ranged: | yes |
| Used By: | United Kingdom Australia Austria Canada Finland Israel Jordan Lebanon South Africa |
| Wars: | Korean War Vietnam War Six-Day War South African Border War |
| Type: | Tank gun |
| Service: | 1948–1976 |
| Caliber: | 84mm |
| Cartridge: | 84×618mm R |
| Part Length: | 226.4inches, 66.7 calibres |
| Elevation: | +18° to −10° in Centurion Mk 5 |
The Ordnance QF 20 pounder (known as 20 pounder, 20 pdr or simply 20-pr) was a British 84 mm (3.307 inch) tank gun. It was introduced in 1948 and used in the Centurion main battle tank, Charioteer medium tank, and Caernarvon Mark II heavy tank. After the 20 pounder gun was found to have inadequate performance against the Soviet T-54 the gun was mostly replaced in service by the larger calibre 105 mm L7 gun.
The gun was developed by the Royal Ordnance Factories.
As fitted to the Charioteer, it ran through two models:
The L7 105 mm tank gun was developed from the 20 pounder. In 1954, the original version of the 105 mm was made by re-boring the tube of a 20 pounder barrel.
The gun was fitted predominantly to the Centurion tank, first seeing action in 1950 with British Army units during the Korean War and Suez Crisis (1956).
In 1956, detailed intelligence on the then-new Soviet T-54A main battle tank was obtained by the British military, after Hungarian rebels drove to the British embassy in Budapest, during the Hungarian Revolution of 1956. Analysis of the T-54's armour suggested that the 20 pounder would be ineffective at penetrating the latest Soviet armour. The 100 mm gun wielded by the T-54 was also assessed. The Centurion's earliest combat experiences and intelligence on Soviet armour and tank guns led to development of the 105 mm L7 tank gun, which was designed to fit specifically into the turret mountings of the 20 pounder, facilitating retrofitting to existing tanks.[1]
During the development of its successor, the 20 pounder continued in front-line service and was even trialed in other fighting vehicles. One was fitted to a Swiss pre-production Panzer 58, replacing a domestic 90 mm Kanone 1948 gun. (However, the Panzer 58 was later equipped with the 105 mm L7.)
Between 1968 and 1971, Mk III Centurions of the Australian Army, equipped with the original 20 pounder, saw action during the Vietnam War. In the context of counterinsurgency operations in South Vietnam, the 84 mm ammunition of the 20 pounder was considered suitable for the armoured fire support role. (North Vietnamese tanks did not operate in South Vietnam until later stages of the war.)
The 20 pounder's APCBC projectile had an initial muzzle velocity of 1,020 metres per second and could penetrate 210mm of rolled homogeneous armour (RHA). However, these conventional rounds were rarely used.
The APDS projectile had a muzzle velocity of 1465m/s and the APDS Mk.3 could penetrate 330mm of RHA at a distance of 1000yd, and 290mm of penetration at 2000yd, equating to a line of sight penetration of 330 mm and 290 mm respectively. Against sloped armour, the APDS had reduced effectiveness: penetrating 87mm and 77mm of RHA at 1000yd and 2000yd respectively, against a plate angled 60 degrees from the normal, this is only 174mm, and 154mm of line of sight penetration. At given ranges, the 20 pounder APDS Mk. III shot only had 53% of its line of sight penetration against a sloped plate, compared to at the normal. Line of sight penetration refers to a flat line drawn through a piece of sloped armour, indicating the effective thickness.
The 20-pounder could also fire high-explosive and canister shot shells.
| Designation | Weight, complete round | Projectile weight | Muzzle velocity | |
|---|---|---|---|---|
| Shot Mk. 1 | 20.6 kg[4] | / | 1000m/s or 1020m/s | |
| APDS Mk. 1 | 16.4 kg | 4.52 kg (3.4 kg without sabot)[5] | 1325m/s | |
| APDS-T Mk. 3 | 15.1 kg | / | 1430m/s or 1465m/s[6] | |
| Shell Mk. 1 | 16 kg | / | 600m/s or 602m/s | |
| SMK BE Mk. 1 | 18 kg | 9.3 kg | 251m/s | |
| CAN | 22.5 kg | 9.5 kg | 910m/s |
