NAME – 13.9x99mmB

NAME (COMMON) – .55 Boys Mk I

TYPE – Belted bottleneck centerfire

YEAR OF INTRODUCTION – 1937

COUNTRY OF ORIGIN – Great Britain

CASE TYPE – Rimless Belted Bottleneck

CARTRIDGE LENGTH – 13.34 cm (5.253 in)

CASING LENGTH – 9.91 cm (3.900 in)

CASE HEAD DIAMETER – 20.14 mm (0.793 in)

CASE BELT DIAMETER – 21.62 mm (0.851 in)

CASE RIM DIAMETER – 20.32 mm (0.800 in)

BULLET TYPE – Full metal jacketed armor piercing

BULLET DIAMETER – 14.27 mm (0.562 in)

BULLET WEIGHT – 60 g (926 grains) Mark I

POWDER WEIGHT – 12.6 g (194.45 grains) [Cordite]

CASING WEIGHT – 58.8 g (2.07 oz)

TOTAL ROUND WEIGHT – 131.4 g (4.63 oz)

MUZZLE VELOCITY – 761 m/s (2495 fps)

NOMINAL BARREL LENGTH – 91.4 cm (36 in)

MUZZLE ENERGY – 17,357 j (12,802 ft/lb)

Mk II AP Bullet 48.02 g (741 grains) AP @ 945 m/s (3100 fps)