.55 Boys
| .55 Boys | ||||||||||||
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| Type | Anti-tank rifle cartridge | |||||||||||
| Place of origin |
 United Kingdom
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| Service history | ||||||||||||
| In service | 1937–1945 | |||||||||||
| Used by | United Kingdom Commonwealth of Nations Finland, et al. |
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| Wars |
WWII Winter War Continuation War |
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| Production history | ||||||||||||
| Designed | 1937 | |||||||||||
| Manufacturer | Kynoch | |||||||||||
| Specifications | ||||||||||||
| 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 | ||||||||||||
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Test barrel length: 914.4 mm (36 in) Source(s): Ammo Encyclopedia, 2nd Edition by Michael Bussard |
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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.
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.
Development on what is known as the .55 Boys was started by Captain H C Boys, a designer at the Royal Small Arms Factory, Enfield. The .55 Boys was designed to be a modified .50 BMG round necked up to accept a larger, steel core bullet in order to increase its armor penetration. A belt was also added to ensure the round could not be chambered in arms designed for the .50 BMG.
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