.55 Boys

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.55 Boys
Type Anti-tank rifle cartridge
Place of origin United Kingdom
Service history
In service 1937–1945
Used by United Kingdom
Commonwealth of Nations
Finland, et al.
Winter War
Continuation War
Production history
Designed 1937
Manufacturer Kynoch
Parent case .50 BMG
Bullet diameter 14.30 mm (.565 in)
Neck diameter 15.392 mm (.606 in)
Shoulder diameter 15.34 mm (.604 in)
Base diameter 20.168 mm (.794 in)
Rim diameter 20.244 mm (.797 in)
Rim thickness 2.44 mm (.096 in)
Case length 97.79 mm (3.85 in)
Overall length 133.43 mm (5.253 in)
Ballistic performance
Bullet mass/type Velocity Energy
946 gr (61 g) Mark I 760m/s (2,495 fps) 17,726 joules (13,074 ft-lbs.)
741 gr (48 g) APCR Tungsten 945m/s (3,100 fps) 21,434 joules (17,726 ft-lbs)
Test barrel length: 914.4 mm (36 in)
Source(s): Ammo Encyclopedia, 2nd Edition by Michael Bussard

The .55 Boys (13.9×99mmB in metric) is an anti-tank cartridge used by the United Kingdom in World War II. It was designed for use with the Boys Anti-Tank Rifle.


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 German 7.92×94mm Patronen and the Soviet 14.5×114mm rounds and, as a result, it was quickly deemed obsolete.[1]


The concept of a small arm round for use against tanks began with the German 13.2mm TuF round, designed during World War I for use against British armour.

In the 1930s, the United Kingdom began designing anti-tank rifle to counter enemy armoured vehicles in the event of a war. Early work on a 13.2mm round was started as a base, likely influenced by the first mass-produced anti-tank cartridge, the 13.2mm TuF, used a 13mm caliber bullet. However, the idea of a 13.2mm round was eventually abandoned.[2]

Development on what is known as the .55 Boys was started by Captain Henry C. Boys, a designer at the Royal Small Arms Factory, Enfield. Boys passed away before the rifle was officially adopted. The .55 Boys was a modified .50 BMG round necked up to accept a larger, steel core bullet in order to increase its armor penetration. A belt was added to reinforce the case with the heavy propellant charge. It is incorrectly reported that the belt was added to ensure the round could not be chambered in weapons designed for the .50 BMG. That is incorrect as the .55-caliber cartage could not be chambered in a .50-caliber weapon to begin with.

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.[1] However, the United Kingdom had to rely on the .55 Boys round for several years because no better infantry anti-tank weapons were available. When the PIAT anti-tank weapon was introduced in 1943, the shaped charges it fired proved to be far more effective against enemy armor than the .55 Boys round had[1] The Boys rifle was phased out of service 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 theaters and also saw use during the Winter War and Continuation War by Finland. By the conclusion of World War II, the .55 Boys was no longer used in any major capacity.


The .55 Boys round went through two major variants in its lifetime, along with an experimental variant that was never adopted by the United Kingdom.

Mark I[edit]

This is the first variant of the .55 Boys. It uses a 926 gr. hardened steel core bullet with a lead sleeve, which is covered with a steel jacket. A ball and tracer version of this round were also created along with a practice round using an aluminum core was also produced in order to be more feasible for training. It has a muzzle velocity of roughly 747 m/s (2,450.1 ft/s).

Mark II[edit]

An improved loading named the Mark II was released in order to increase the round's velocity and its penetration. It generates a muzzle velocity of approximately 884 m/s (2,899.5 ft/s).[3]

At an ideal angle, the Mark 2 round was able to pierce .91 inches (23.2mm) of armor at 100 yards, .82 inches (20.9mm) at 300 yards and .74 inches (18.8mm) at 500 yards.[4]

APCR Tungsten Round[edit]

An experimental APCR (Armor Piercing Composite Rigid) .55 Boys round was designed in 1942.[5] It used a tungsten core round instead of a steel core, which greatly increased its penetrating ability and gave a boost to its muzzle velocity from the Mark II's 884 m/s to approximately 944 m/s (3100 ft/s). It differs from the Mark I and II rounds because of its two-part bullet. This model was never officially adopted because far better anti-tank rounds and weapons, such as the PIAT, were entering service at the time. The .55 Boys, even with a greatly improved bullet, was simply too weak to defeat the tanks being fielded by the Axis powers.


See also[edit]


External links[edit]