The SP-4 cartridge was a step forward in developing special captive-piston ammunition for silent weapons. The piston bears up directly against an elongated blunt-nosed bullet. The bullet is a hard alloy cylinder having a brass driving band secured to its top. The driving band engages the bore rifling, thus imparting spin to the bullet. By the time the bullet completely leaves the case, its driving band reaches the muzzle. The piston fully stays in the cartridge after the shot is fired, so the length of the spent case does not exceed the original cartridge length. The noise level» generated by a PSS shot, is somewhere between a shot of a 4.5 mm air rifle (101 dB) and a clap. The heavy 10-g bullet penetrates Class 2 body armor or a steel army helmet at a range of 25 m.
A considerable reduction in the spent case length allowed the designers to implement the self-loading principle with automatic extraction of the spent case from the chamber by the recoiling slide. However, the powder gas pressure inside the cartridge case remains high after the pistol is fired, thus, self-loading was quite an achievement, which demanded special measures to be taken. The PSS pistol features a unique blowback-operated design with a moving chamber — the latter moves somewhat to the rear along with the recoiling slide. The fact that the rifled bore is separated from the chamber prevents the creation of negative pressure behind the bullet (expanding powder gases do not escape into the bore), which otherwise may have resulted in an audible pop. Besides, extraction of the spent case is made easier. The recoil spring is wound around a frame peg above the barrel. The front end of the slide carries a stop in the form of a rotating sleeve with recesses for fingers. The extractor is mounted on the right side of the slide.
The trigger assembly is single-action or double-action with an external hammer, and there is a slide-mounted manual safety. To ensure commonality and cut the development and production costs, the trigger assembly was generally adopted from the standard Makarov PM pistol. Like with the PMa the safety catch, when engaged, releases the hammer safely from full cock and puts into half-cock. The lower end of the hammer spring serves as the magazine release.
A solid plastic grip as screwed to the frame. The pistol is fed from a detachable single-column box magazine- The PSS pistol grip is larger than that of the PM. due to a longer cartridge. The PSS is superior to the non-automatic MSP and 3-4M both in the effective rate of fire and muzzle energy, whole featuring the same muzzle report suppression level.
The PSS, designed to replace the PB pistol in military units, is manufactured by the Tula Arms Plant. The SP-4 cartridge is also used in the firing device of the NRS-2 special scout knife and in the OTs-38 special revolver.