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During the mid-1970’s the Russians developed a unique smoothbore underwater assault rifle, the APS (Автомат Подводный Специальный).

These cartridges were manufactured in 3 types, namely MPS, Morskoy Patron Spetsialnyy; “special marine cartridge” (МПС; морской патрон специальный) and MPST, Morskoy Patron Spetsialnyy Trassiruyushchiy; “special marine tracer cartridge” (МПСТ; морской патрон специальный трассирующий). The final is a training cartridge of unknown designation. Whilst the APS is commonly referred to as an assault rifle, the barrel of the Avtomat Podvodny Spetsialny or “special underwater assault rifle” is smooth bored, and the projectiles are stabilised by hydrodynamic effects.

The MPS and MPST cartridges feature long, slender dart-like projectiles of a nominal 120 mm length. The projectiles are of lacquered steel, with the MPST projectile featuring a tracer canister at its base. The lacquered steel cartridge cases are sealed at the case mouth and the primer annulus to prevent the ingress of moisture.

Whilst there is little public domain information on these cartridges, the following specifications (for the MPS cartridge) and diagrams are taken from the Russian language book Ammunition For Small Arms, by L.V. Kolomiytsev, V.T. Nikitiuk, I.S. Sobakar, and V.V. Somov,published in Ukraine in 2003.

МПС (MPS) Cartridge Specifications

Calibre: 5.66 x 39
Cartridge length: 149.6 – 150.0 mm
Cartridge weight: 27.5 – 28.0 g
Muzzle velocity when fired from APS above water: 350 – 365 m/s
Muzzle energy when fired from APS above water: 1237.3 – 1365.6 J

Propellant type: smokeless pyroxyline
Weight of powder charge: 1.40 – 1.42 g
The maximum pressure of the powder gases: 295.9 MPa

Lacquered steel projectile
Bullet weight: 20.2 – 20.5 g
Bullet length:  119.8 – 120.0 mm

Cartridge case
Weight of cartridge case with primer: 5.50 – 5.57 g
Cartridge case length: 39.5 – 39.7 mm
Material: lacquered steel

The APS was developed as a combat diver weapon intended as a personal weapon for usage under water mainly against enemy divers. The APS Underwater Assault Rifle has a unique design which has no analogues except later Russian developments of the improved underwater assault rifle. The Russian Federation is the only country whose combat divers are currently armed with the APS Assault Rifles. The weapon is produced by Tula city firearms plant.

Until the development of the APS Russian combat divers were armed with usual firearms which could not be used under water. Combat divers used firearms mostly for landing operations. Usual firearms were carried under water to landing points and used only afterwards when divers reached shore. While under water the combat diver could use only knives and was as defenceless as any normal soldier

Underwater Assault Rifles are based on a gas returning principle through the gas chamber. Its automatic selector allows it to execute fire on land or under water in single or automatic fire modes. In auto mode the APS executes short 3 - 5 shot bursts or a continuous 10 shot burst. Overall the APS Special Assault Rifle has very much in common with the AK-74 Assault Rifle, but with lighter dimensions and weight comparing with the later mentioned assault rifle.

The APS fires a 120 mm length dart bullet. The dart has a weight of 26g and is stabilized under the water by a cavitation effect. On land this bullet isn't stabilized. The APS can fire a standard 5.45 x 39 mm Russian cartridge by changing its smoothbore barrel into the rifled one chambered for the 5.45-mm. The Rifle is fitted with a 26 round magazine. The magazine has an uncommon shape determined by the long cartridge. The magazine is divided into two sections to prevent reloading failures.