NAME – 12.7x133mm Mark 59 Underwater Projectile
NAME (COMMON) – Mark 59 Underwater Projectile
TYPE – Captured piston flechette
YEAR OF INTRODUCTION – 1967
COUNTRY OF ORIGIN – USA
CASE TYPE – Straight rimless
CARTRIDGE LENGTH – 13.3 cm (5.24 in)
CASING LENGTH – 13.3 cm (5.24 in)
CASE HEAD DIAMETER – 12.7 mm (0.50 in)
BULLET TYPE – Fin-stabilized Flechette
BULLET DIAMETER – 2.54 mm (0.10 in) x 108 mm (4.25 in), 3.18mm (0.125 in) across fins
BULLET WEIGHT – 9.91 g (153 grains)
TOTAL ROUND WEIGHT – 0.102 kg (1,575 grains – 3.6 oz)
MUZZLE VELOCITY – 226 m/s (740 fps) in air
MUZZLE VELOCITY 210 m/s (690 fps) at 18.3 m (60 foot) Depth
NOMINAL BARREL LENGTH – n/a
MUZZLE ENERGY – 252 j (186 ft/lb) in air
MUZZLE ENERGY 219 j (162 ft/lb) at 18.3 m (60 foot) depth
The Mark 59 underwater projectile is a round of ammunition made solely for use in the Mk I Underwater Defense weapon. The round itself is a smooth steel cylinder with a small indentation at the rear where the primer is, and the muzzle end is sealed with a metal cup. On the side of the cylinder is a small machined flat for securing the round in place in the cylinder of the firing weapon.
Mk.59 underwater ammunition is based on a cylindrical steel case about 5.5 inches (14 cm) long. Rear part of the case contains primer and small powder charge behind a cup-shaped piston. About 2/3 of the case length forward of the piston is used to contain a needle-like projectile, made of tungsten alloy, which is held centrally by several plastic discs. Projectile diameter is 0.1 inch (2.54mm) and length is about 4 inches (10 cm). Projectile weight is 9.9 gram; muzzle velocity is about 225 meters per second. Projectile (bullet) is stabilized in water by four tail fins, which are slightly inclined to provide rotation. Effective range under water is about 10 meters at 18 meters depth. Upon discharge, powder gases violently accelerate piston forward, so it pushes projectile out of the case and toward the target. At the mouth of the case piston is stopped by annual constriction, built into the case, to capture powder cases inside the case. As a result, there’s no visible muzzle flash, no gas bubble when firing underwater and almost no sound (and associated shock wave) of gun firing.
