This design allows a gun to be much quieter without a suppressor. It's fairly simple in its concept. When it's fired the blast (and noise) is on the rear side of the red piston. Next the piston is propelled towards the case mouth, pushing the projectile (gray cylinder) out of the casing, through the barrel, and into to the target. The piston remains in the casing, crumpling and forming an air tight seal.
~~This Soviet concept to my knowledge doesn't have a NATO equivalent~~ there is something similar if you search up Quiet Special Purpose Revolver. Despite the fact it's reportedly only 110 db loud, roughly as loud as a silenced 22lr.
Three firearms models have been chambered in it the NRS-2 knife, OTs-38 revolver, and PPS pistol. (Shown in order)
Here is it being fired out of the NRS-2 knife gun:
https://youtu.be/vW3ZBLlPz_c?si=UCoCEwzaqVANMcY_ [00:11]
It's simply a really clever approach to a design requirement, but of course there's at least three reasons it's not more popular.
One, the piston system must increase the cost to produce each round.
Two, The ATF treats these as silencers legally. This means each shot come with a $200 tax for anyone in the world's largest civilian firearms market.
And three, how often does a government or individual need to fire a silenced shot but can't bring along a suppressor? Is this a solution looking for a problem?
Also heres some more stuff about the NRS-2:
This is pretty cool, but what kind of muzzle velocity or impact energy would these rounds have?
Part of the reason for longer barrels is to let the powder burn and fully push the round. A subsonic round would probably give similar performance on a traditional cartridge.
If you're concerned with noise, a suppressed .22 is quieter than a pebble thrown in the woods.
I found one source https://dockeryarmory.com/7-62x42mm-sp-4/ claiming they are 153 grain bullets moving at 621 fps at the muzzle. So they are subsonic.
It also claims 133ft/lbs at the muzzle and the low end of lethal is supposedly ~58ft/lbs. Sub sonic 22lr is ~90ft/lbs.
One other cool thing about this though is that it doesn't need the length of a barrel (beyond what it needs to stabilize) because it isn't gaining anymore velocity after the piston hits the bullet.