.55 Boys

.55 Boys
TypeAnti-tank rifle cartridge
Place of originUnited Kingdom
Service history
In service1937–1945
Used byUnited Kingdom
Commonwealth of Nations
Finland, et al.
WarsWorld War II
Winter War
Continuation War
Production history
Designed1937
ManufacturerKynoch
Specifications
Parent case.50 BMG
Case typeBelted, bottleneck
Bullet diameter14.3 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 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.