Prototype Underwater Machinegun - AG-026 ( lemmy.world )

AG-026 is a simple/direct blowback firearm firing from an open bolt with a fixed firing pin. The bolt itself is fairly compact and low profile with a small cross-sectional area for decreased resistance while cycling in water, however, it is linked to а pair of rotating gears/flywheels enclosed in the circular housings mounted to either side of the breech area. The gears increase the reciprocating mass to make the blowback action work properly. The gun was charged by rotating the handles located on the circular gear housings.

https://lemmy.world/pictrs/image/cde33d5b-3336-4052-a863-9cda7ccabd71.png

The AG-026 machine gun is chambered in 5.66x39mm, the same cartridge used in the APS rifle. This cartridge is based on the 5.45x39mm case, has a long 120mm (4.7″) projectile designed for stabilizing underwater, enhanced primer and neck seal as well as anti-corrosive coating on the case and bullet. The main load of 5.66x39mm cartridge is called MPS (МПС). Its 20 gram (308 grains) projectile has a muzzle velocity of 365 m/s (1200 fps). The effective range of this bullet fired from AG-026 is 30 meters at a 5-meter submersion depth, 10 meters at a 40-meter depth and up to 100 meters in the air.

https://lemmy.world/pictrs/image/e5dfd818-dbaa-4eb2-8c2f-c2f9b039efea.jpeg

The AG-026 machine gun was fed from a 27-round rotary magazine that represents a rotating closed-chain ammunition belt placed into an outer protective shell. The gun has a push feed action where the bolt simply pushes the aligned cartridge from the link to the chamber. The only control is apparently the safety selector on the right side of the receiver with safe and full-auto modes. The overall weight of this machine gun is 4.85 kilograms (10.7 lbs)...
https://www.thefirearmblog.com/blog/2020/09/30/soviet-ag-026-prototype-underwater-machine-gun/