7.62x42 blowback, double-action, external hammer, 6-rnd box mag. The cartridge has a piston between bullet and propellant. The propellant explodes, drives the piston forward to impell the bullet; the piston is arrested by the cartridge shoulder. The noise is retained inside the cartridge case.
Effective range is about 50 meters.
Its a wonderful wacky world. You think this is good, the Germans have an underwater pistol, the P-11, 7.62 x 36 which fires darts ignited electronically from a battery in the pistol grip. Heckler & Koch is thought to be the maker but they've never officially claimed responsibility.
The Russians have an underwater assault rifle, a gas operated 5.66mm selective fire with a 26 rnd box mag. The projectiles are 120mm long drag stabilized darts fired from conventional cartridge cases. It can be used above water, too. And fun for the swimming pool.
Typical variation of the old "spigot-launcher" type round. Research the PIAT and the spigot mortar and the "Hedge Hog" launcher.
"How does the case retain the gasses without building preasure and blowing up the gun?"
Look at the listed muzzle velocities.
High peak impulse of short duration. Sustained peak pressures are what blow cases.
A short, sharp shot to accelerate the projectile. Also the steel cases are re-inforced at high stress areas.