The .55 Boys is a .50 BMG cartridge necked up to accept a .55 caliber bullet and with a belt added to its case. It performed poorly when compared to contemporary foreign anti-tank rounds, such as the German7.92×94mm Patronen[note 1] and the Soviet14.5×114mm rounds and, as a result, it was quickly deemed obsolete.
In the 1930s, the United Kingdom began designing an anti-tank rifle to counter enemy armoured vehicles in the event of a war.
Initially the gun design was trialled using .50 inch bullet with a belted case due to lack of armour-piercing performance the calibre was increased to .55 
Development on what is known as the .55 Boys was started by Captain Henry C. Boys, the Assistant Superintendent of Design at the Royal Small Arms Factory, Enfield in 1934. Boys died before the rifle was officially adopted and it was named after him. The .55 Boys round was a modified .50 BMG round necked up to accept a larger, steel-cored bullet in order to increase its armour penetration. A belt was added to reinforce the case with the heavy propellant charge.[note 2]
The .55 Boys was adopted and manufactured alongside the Boys Anti-Tank Rifle in 1937 throughout the Commonwealth of Nations by firms such as Kynoch. However, when the United Kingdom entered World War II, the .55 Boys round was soon found to be insufficient against even early war Axis tanks in late 1939 and 1940. However, the United Kingdom had to rely on the .55 Boys round for because no better infantry anti-tank weapons were available. When the PIAT anti-tank weapon was introduced in 1943, the shaped charges it fired were far more effective against enemy armour than the .55 Boys round had been. The Boys rifle was phased out of service on the frontline as the PIAT became the British military’s primary handheld anti-tank weapon. Despite its lack of effectiveness as an anti-tank weapon, the .55 Boys was used throughout World War II in both the Pacific and Atlantic theatre and also saw use during the Winter War and Continuation War by Finland. The Boys was issued to Home Guard units in the UK for use against “light armoured fighting vehicles…which the Home Guard are likely to have to deal with, certainly in the early stages of either an air-borne or sea-borne landing on our coasts.” A handbook for its use noted that as well as the expected penetration of armour at various distances and angles that it would penetrate 14 inches of brick wall and 10 inches of sandbags.
By the conclusion of World War II, the .55 Boys was no longer used in any major capacity.