In the early 1970s, the Soviets developed their APS Underwater Assault Rifle:
The APS was intially based on the AK-74 rifle, but several significant changes were made. It fires a 120mm long 5.6mm dart or flechette, which is relatively stable traveling underwater — it uses a smoothbore barrel and relies on the shape of the dart for stability and accuracy. The dart is fired by a standard (although well waterproofed) cartridge case with powder and primer. The magazine shape is dictated by the long projectiles, and holds 26 rounds.
The rifle uses a gas piston like the AK to operate, but fires from an open bolt. This ensures that the barrel remains filled with water, which is necessary to properly stabilize the projectiles. It can be fired dry above water, but this is inaccurate and causes significant wear to the gun. The trigger mechanism has a selector for semi or full automatic fire, and a collapsing wire buttstock provides some stability.
The APS is by far the most fully developed underwater firearm in use, although it does have shortcomings. The wide magazine profile can make maneuvering in water somewhat difficult, and the sights are simple and crude.
Because of increasing water pressure, the APS performs differently at different depths. As one submerges farther, the cyclic rate of the weapon slows down and the muzzle velocity and effective range decrease. At 5m depth, the APS is considered to have an effective range of 30m, decreasing to 11m at a depth of 40m. This change in pressure also necessitates a self-adjusting gas system, to ensure that the action cycles reliably in different pressures. The notion of designing an effective underwater firearm really bring to light a number of interesting considerations not relevant on normal guns.
(Hat tip to Borepatch, who pointed to the Forgotten Weapons site.)
Don’t forget the SPP-1 by the same designer, a four-barreled underwater pistol, more details in this Guns.com article or that Defense Journal article.
It also didn’t stop there and then, they developed several successors, in the nineties the ASM-DT and more recently, in 2007, Tula developed the ADS, a modernized new design.
The ADS is capable of operating in two environments, under water, using a thin sabot bullet, which unlike the darts, is based on the 5.45mm cartridge, this newer round does not need a special rifle with an over-sized magazine. Here is a picture of the ASP (top) and ADS (bottom) rounds compared side by side.
Outside in the air, it discharges standard cartridges of the same 5.45mm caliber.
And in the end, a short video presentation of the Tula ADS!