The M1 gun, also called the Mark 1 Mod 0 Underwater Defense Gun, is a firearm developed for underwater use by the United States military during the height of the Cold War with the Soviets. Like most other underwater guns, it was designed to fire a longer, streamlined projectile rather than a bullet.

The M1 ammunition was engineered as a unique 4.25 inch dart made from a tungsten alloy. This was cataloged separately as Mark 59 underwater ammo. The projectile was a needle-like 0.1" diameter and weighed less than 10 grams. It also had four tail fins to ensure a slight rotation.

History The firearm was designed as part of a project which was carried out in the 1960s at the White Oak Laboratory of Silver Spring, MD. Labeled TDP 3801, the original goal of the project was the LanceJet, a highly portable rocket-firing weapon. However, the LanceJet proved to be both inaccurate and overly expensive to produce. Focus then shifted to the M1 as a counter to the Soviet Navy’s SPP-1 underwater gun. The US M1 met with the approval of the Naval Surface Weapons Center and went into official use in the early 1970s.

Loading and Firing The Mark 1 Mod 0 Underwater Defense Gun is a pepper-box type firearm. The removable cylinder held six projectiles held in position with plastic disks. Most Navy personnel who carried the weapon also carried an extra loaded cylinder, which could be reloaded directly into the gun through a spring-loaded door on it's left side. Read the rest of the Underwater guns here.

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 with spare cylinder. Photo Joe Trevithick

Diagram from patent issued to AAI Corp for captive piston cartridge for underwater gun

Trigger type	Double action only
Caliber	.1 (2.54mm)
Weight	1.02 kg empty, 1.89 kg loaded
Length	248 mm
Barrel length	-
Capacity	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 1/6^th 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’clockposition. 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.

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