.55 Boys

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.

.55 Boys
TypeAnti-tank rifle cartridge
Place of originUnited Kingdom
Service history
In service1937–1945
Used byUnited Kingdom
Commonwealth of Nations
Finland, et al.
Winter War
Continuation War
Production history
Parent case.50 BMG
Bullet diameter14.30 mm (.565 in)
Neck diameter15.392 mm (.606 in)
Shoulder diameter15.34 mm (.604 in)
Base diameter20.168 mm (.794 in)
Rim diameter20.244 mm (.797 in)
Rim thickness2.44 mm (.096 in)
Case length97.79 mm (3.85 in)
Overall length133.43 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 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]


.A 55 Boys cartridge (left) and a .50 BMG cartridge (right).

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 the first British tanks.

In the 1930s, the United Kingdom began designing an anti-tank rifle to counter enemy armoured vehicles in the event of a war. Early work on a 13.2 mm (0.52 in) round was started as a base, likely influenced by the first mass-produced anti-tank cartridge, the 13.2mm TuF, used a 13 mm calibre bullet. However, the idea of a 13.2 mm 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 died before the rifle was officially adopted. The .55 Boys 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.[3]

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 armour 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 theatre 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 IEdit

This is the first variant of the .55 Boys. It uses a 926 gr (60.0 g). hardened steel core bullet with a lead sleeve, which is covered with a steel jacket. A ball and tracer version of this round was also created along with a practice round using an aluminum core in order to be more feasible for training. It has a muzzle velocity of roughly 747 m/s (2,450 ft/s).

Mark IIEdit

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).[4]

At an ideal angle, the Mark 2 round was able to pierce 0.91 inches (23.2 mm) of armour at 100 yd (91 m), 0.82 inches (20.9 mm) at 300 yards and 0.74 inches (18.8 mm) at 500 yards.[5]

APCR tungsten roundEdit

An experimental armour-piercing composite rigid (APCR) .55 Boys round was designed in 1942.[6] It used a tungsten core 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 alsoEdit


  1. ^ a b c Ammo Encyclopedia, Second Edition by Michael Bussard, p. 563
  2. ^ "Enfield – BSA "Boys" Anti-Tank Rifle". Archived from the original on 2011-11-01. Retrieved 2012-03-26.
  3. ^ It is incorrectly reported that the belt was added to ensure the round could not be chambered in weapons designed for the .50 BMG. though .55-calibre cartage could not be chambered in a .50-calibre weapon to begin with.
  4. ^ Bad Boys The British Rifle, Anti-Tank, .55 In, Boys[self-published source]
  5. ^ Small Arms Training Volume 1, Pamphlet No. 5 Anti-Tank Rifle 1942, HMSO
  6. ^ "An Introduction to Anti-Tank Cartridges" by Tony Williams