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:
Depends on a bunch of things like the volume of the suppressor, the gun it is mounted on etc. Theoretically it should be quieter as any escaping expanding gas is bled off, including the air around the projectile in front of the piston as it gets superheated and compressed etc. I doubt it would be noticeably quieter to the human ear as I'm guessing the rounds are already subsonic out of all the guns that use them, but I could be wrong.
There are a couple potential side effects though. It is possible the silencer decreases the ballistic performance significantly. I'm going to guess these rounds are barely lethal in their stock configuration so even a 10-20% drop in velocity could be a big hindrance to their effectiveness. Also suppressors tend to increase back pressure so it seems possible that could mess with the function of these rounds. Would it delay the piston coming forward, or possibly blow the primers out the back of the rounds? Hard to say without testing but it is a possibility.
~~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.