Ref the gas seal, the cylinder does slide forward to cut down (not eliminate) the gases escaping from the gap between the cylinder and barrel.
How do you figure?????
The 7.62 Nagant round is longer than the cylinder. When the cylinder slides forward, the end of the case mouth is shoved into the forcing cone.
When the cartridge is detonated, the bullet, as it is propelled forward, expands the case mouth against the forcing cone, creating a complete gas seal.