Russian SP-4 Silent Ammunition
The SP-4 is a unique type of cartridge. It builds on the Russian "piston driven" projectile design of older cartridges, and uses a "piston" to seal the gases within the cartridge. Therefore, no gases escape the barrel when fired, resulting in a silent discharge.
The piston seals against the case bottleneck. Presumably the gases slowly leak from the spent cartridge over time. The amount of powder beneath the piston is relatively small, as show by the cutout images. The tip of the piston centers the trailing end of the bullet.
The bullet appears to be no more than a slug of the wadcutter design with a brass/copper sleeve to align against the rifling:
Spent cartridge on the right, revealing the top of the piston:
Three distinct applications are made from this design - the first obviously being clandestine/SOF applications where silence is paramount. To this end, the Russians designed the SPP pistol for the ammunition. The pistol has a floating chamber with a 2nd buffer spring to eliminate metal-to-metal slap against the barrel and slide.
The second application is for survival/escape type scenarios where silence is key. The Russians developed a survival knife (presumably for downed pilots) that has a small gun in the handle.
Lastly, since there is no gas discharge, a number of construction- and field-demolition applications can utilize this cartridge as the basis for certain tooling. The examples in use are chain cutting tools that fire a round against a piston that cuts the chain, and rivet drivers similar to nail guns that use the round to drive a large fastener into the target... with underwater demolition, such a use would be to attach a demolition charge to an underwater mine without the use of magnets. Other uses can surely be found for this type of cartridge.
Caliber, mm 7.62 x 41.5