…today we have a chance to take a look at one of the Soviet experimental sniper rifles/DMRs chambered in the 6x49mm caliber. The rifle is called TKB-0145S (ТКБ-0145С)…

The 6x49mm was a high-pressure cartridge launching 6mm projectiles at a high muzzle velocity of 1,150 m/s which is about 3,770 fps… With such a high muzzle velocity, it was an extremely flat shooting cartridge. The bullet drop was minimal resulting in a long point blank range and the projectiles were also less affected by the wind…

As you can see in the images, TKB-0145S is not only a bullpup rifle but also has a possibility to fold in half resulting in a folded length of 32.8″ (834mm) at a barrel length of 28.3″ (720mm). The hinge point of the rifle is located in front of the rear sight base. There is a spring-loaded latch on the barrel which catches the lug located on the rear on the receiver thus retaining the rifle on its folded position. Folding of the rifle does not require removal of the scope and the magazine. This feature decreases the time required to prepare the rifle for firing.

The TKB-0145S rifle is gas operated with a rather interesting gas system. There is no gas port drilled into the barrel. The gasses are vented into the system after they have left the muzzle. This is somewhat similar to the Bang system in terms of the general principle. This design solution was used to eliminate any possible effects of the gas system onto the bullet’s flight…

https://www.thefirearmblog.com/blog/2018/07/09/soviet-experimental-6mm-sniper-rifle-tkb-0145s/