.55 Boys

.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.
Wars	World War II Winter War Continuation War
Production history
Designed	1937
Manufacturer	Kynoch
Specifications
Parent case	.50 BMG
Case type	Belted, bottleneck
Bullet diameter	14.3 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 760 m/s 17,726 J 741 gr (48 g) APCR Tungsten 945 m/s 21,434 J
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 during World War II. It was designed for use with the Boys anti-tank rifle.

The .55 Boys is a 13.2×99mm Hotchkiss cartridge, necked up in the mid-1930s to accept a .55 caliber bullet.^[1] Since the shoulder of the case was narrowed, a belted magnum belt was added to ensure reliable headspacing. The .55 Boys performed poorly compared to contemporary foreign anti-tank rounds, such as the German 7.92×94mm Patronen^{[note 1]} and the Soviet 14.5×114mm rounds. As a result, it was quickly deemed obsolete.^[2]

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

In the 1930s, the United Kingdom began designing an anti-tank rifle to address the threat posed by enemy armored vehicles in the event of a war.

Initially, the gun design was trialed using a .50 inch bullet with a belted case. Due to insufficient armor-piercing performance, the caliber was increased to .55.^[3]

Development of the .55 Boys was initiated 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 in his honor. The .55 Boys round was a modified .50 BMG round necked up to accept a larger, steel-cored bullet to increase armor penetration. A belt was added to reinforce the case due to the heavy propellant charge.^{[citation needed][note 2]}

The .55 Boys was adopted and manufactured alongside the Boys anti-tank rifle in 1937 by firms such as Kynoch throughout the Commonwealth of Nations. When the United Kingdom entered World War II, the .55 Boys was soon found to be insufficient against even early war Axis tanks in late 1939 and 1940.^[2] However, the UK had to rely on the .55 Boys due to the lack of better infantry anti-tank weapons at the time. When the PIAT anti-tank weapon was introduced in 1943, its shaped charge proved far more effective against enemy armor than the .55 Boys.^[2] The Boys rifle was phased out of frontline service as the PIAT became the British military's primary handheld anti-tank weapon.

Although not highly effective as an anti-tank weapon, the .55 Boys was used until the end of World War II by British and Commonwealth forces. It 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 armored fighting vehicles... which the Home Guard are likely to have to deal with, certainly in the early stages of either an airborne or seaborne landing on our coasts." A handbook for its use noted that, in addition to the expected armor penetration at various distances and angles, it would penetrate 14 inches of brick wall and 10 inches of sandbags.^[4]

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.

The first variant of the .55 Boys used a 926 gr (60.0 g) hardened-steel-core bullet with a lead sleeve, covered with a steel jacket. A ball and tracer version of this round was also developed, along with a practice round featuring an aluminum core to make it more suitable for training. It has a muzzle velocity of approximately 747 m/s (2,450 ft/s).

An improved loading, known as the Mark II, was introduced to enhance the round's velocity and penetration. It achieves a muzzle velocity of approximately 884 m/s (2,899.5 ft/s).^[5]

At an optimal angle, the Mark II round was capable of penetrating 0.91 inches (23.2 mm) of armor at 100 yd (91 m), 0.82 inches (20.9 mm) at 300 yards (270 m), and 0.74 inches (18.8 mm) at 500 yards (460 m).^[6]

An experimental armour-piercing composite rigid (APCR) .55 Boys round was developed in 1942.^[7] It featured a tungsten core instead of the steel core used in previous rounds, which significantly enhanced its penetrating ability and increased its muzzle velocity from the Mark II's 884 m/s to approximately 944 m/s (3,100 ft/s). This round differed from the Mark I and II rounds due to its two-part bullet design. However, it was never officially adopted, as more effective anti-tank rounds and weapons, such as the PIAT, were being introduced. Despite its improvements, the .55 Boys, even with the enhanced APCR round, was insufficient against the heavily armored tanks of the Wehrmacht. Nevertheless, it remained effective against the more lightly armored tanks of the Imperial Japanese Army in the Asian and Pacific theaters until the end of the war.

• 13mm caliber
• 14.5×114mm
• 12.7×108mm
• List of rifle cartridges