That's some rare and highly specialized silent ammo.
The first noiseless cartridge developed by Stechkin was designated SP-1 (Spetsialnyj Patron 1 – Special Cartridge model 1). It was based on 9×18 case, loaded with special bi-caliber bullet and small piston, which was located between bullet base and powder charge. The bullet had front part of 7.62mm caliber, with base being of 9mm caliber, and it was intended to be fired through the squeeze-bored barrel with decreased caliber muzzle. That way, bullet base was squeezed to the 7.62mm and then free to exit the bore, while the piston was jammed in the bore, capturing powder gases inside the barrel. Of cause, this system was to be used only in manually loaded single-shot weapons, and one such weapon (declassified recently) was made to imitate a tin cigarette case, with three 9 / 7.62mm barrels and firing mechanism fitted inside.
The SP-1 cartridge never went past prototype stage, but within several years Stechkin designed his next internally silenced cartridge, known as SP-2. This one also contained a small powder charge behind the piston, but this time powder gases were to be captured inside the bottlenecked cartridge case, which looked somewhat similar to that of the 7.62×39 M43 round. Since the pusher piston was relatively short, and before discharge seated deep in the cartridge case, the bullet had to be made long, but relatively light. To achieve this, Stechkin used metal jacket taken from an unassembled 7.62mm pistol (7.62×25) bullet and fitted it with aluminum core that extended back from the jacket to rest on the piston.
This round apparently was produced in very limited numbers and used in some ‘Q-style’ weapons, such as the three-shot pistol disguised as a tin cigarette container. Also, it was probably used in the earliest model of the more convenient-looking two-barrel derringer-type pistol, which served as a predecessor to the MSP pistol described below. http://www.forgottenweapons.com/russian-silent-ammunition/
- Integrally silenced ammunition compared to standard Soviet ammunition, L to R: standard 9×18 PM, 7.62 SP-2, 7.62 SP-3 loaded, 7.62 SP-3 fired case (note extended two-stage piston and re-formed case neck), standard 7.62×39 M43, standard 7.62x54R, 7.62 PZAM (loaded round and fired case with extended single-piece piston). Image courtesy of Yuri Bushin.