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Mark 1 Model 0 / Mk.1 Mod.0 Underwater Defense Gun pistol (USA)

Mark 1 Model 0 / Mk.1 Mod.0 Underwater Defense Gun pistol
Mark 1 Model 0 / Mk.1 Mod.0 Underwater Defense Gun with spare cylinder


patent diagram for ammunition for Mk.1 Mod.0 Underwater Defense Gun
patent diagram for ammunition for Mk.1 Mod.0 Underwater Defense Gun

Trigger type

Double action only


.1 (2.54mm)


1.02 kg empty, 1.89 kg loaded


248 mm

Barrel length


6 rounds

Mark 1 Model 0 / Mk.1 Mod.0 Underwater Defense Gun was developed for combat swimmers of US Navy (UDT and SEAL teams) during late 1960s, and entered service in 1970. It was used by US Navy divers until mid-1980s, when it was completely replaced in service by German-made HK P11 underwater pistol.

Mk.1 Mod.0 Underwater Defense Gun is based on the patents, issued to AAI Corporation, specifically US 3,476,048 (for captive piston underwater ammunition) and US 3,729,853 (the gun itself).

Mk.1 Mod.0 Underwater Defense Gun is a double-action only pepperbox-type weapon, made mostly of lightweight alloys. It uses detachable cylinders that hold six rounds. Gun is reloaded by removing empty (fired) cylinder through the hinged, spring-loaded and down-opening door on the left side of the gun casing, and placing inside a loaded one. Loading door latch is located on the left side of the gun, above the grip. Once a new loaded cylinder is placed into the casing and door is closed up, gun is ready to fire. Firing mechanism consist of an exposed, double-action only trigger with manual safety. Each pull of the trigger rotates cylinder through 1/6th of a turn to place a next live round against the firing pin, then cocks and releases the striker to fire the cartridge which is located at the 6 o’clock position. Manual safety is a rotating knob, located behind the trigger. It can be installed on either side of the gun, depending on the user preferences. Gun is fitted with fixed iron sights with three bright yellow dots to assist aiming.

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. During the mission, combat divers usually carried spare loaded cylinders in their gear to facilitate fast reloads, but fired cylinders can be easily reloaded with fresh cartridges when cylinder is removed from the gun.