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:
~~Most models of suppressors will raise muzzle velocity just like adding barrel length does. Unless you have rubber wipes in there.~~ Edit: I realized that because the gases are all trapped in the cartridge it won't continue to extert force through the length of the barrel or suppressor.
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.