Ordnance QF 20-pounder
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. The 20 pounder was designed to counter the T-44 in a possible future war scenario and to replace the Ordnance QF 17 pounder, which had proven itself effective in World War II. However, once 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. Design and development. 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. Service history. The gun was fitted predominantly to the Centurion tank, seeing action with British and Australian forces during the Korean and Vietnam War. When a Soviet T-54A main battle tank was  driven to the British embassy in Budapest by Hungarian rebels during the Hungarian Revolution of 1956, analysis of its armour and 100 mm gun led British officials to determine that the 20 pounder was ineffective at defeating Soviet armour. This led to the development of the 105 mm L7 tank gun, which was designed to fit specifically into the turret mountings of the 20 pounder, facilitating for easily upgunning existing tanks equipped with the 20 pounder. One 20 pounder gun was fitted to a Swiss pre-production Panzer 58, replacing a domestic 90 mm Kanone 1948 gun, before it was  equipped with the 105 mm L7. Performance. The 20 pounder's APCBC projectile had an initial muzzle velocity of 1,020 metres per second and could penetrate of rolled homogeneous armour (RHA). However, these conventional rounds were rarely used. The APDS projectile had a muzzle velocity of and the APDS Mk.3 shell could penetrate of RHA at a distance of , and of penetration at , equating to a line of sight penetration of 330 mm and 290 mm respectively. Against sloped armour, the APDS had reduced effectiveness: penetrating and of RHA at and respectively, against a plate angled 60 degrees from the  normal, this is only , and 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.