A third generation of the PK machine gun
In 1993 a new member of the PK family chambered for the unique 6 x 49 mm round was introduced. Called the Unified Machine Gun (UMG), it is basically similar to the PK/PKM, but has a shorter receiver, apparently with a simplified feed mechanism. The UMG has a longer barrel, while overall length (1150 mm) is approximately equal to the standard issue PK/PKM. This new Russian machine gun is relatively light in weight, as it weighs only 6.5 kg with a bipod. The revealed prototypes have no iron sights, but were fitted with an optical sighting device. In addition to the improved PK machine gun, the SVD based sniping rifle and AK assault rifle chambered for the 6 x 49 mm cartridge are also being developed.
Presumably inspired by the U.S. Army SAW research, the Soviets started developing the 6 mm ammunition primarily intended for infantry machine guns and sniping rifles in about 1980. The first type revealed was the 6 x 53 mm, which launched an 5.2 g bullet with a muzzle velocity of 1150 mps.
About ten years later the 6 x 49 mm cartridge was introduced. It has a rimless case with approximately the same rim diameter (11.4 mm) as a classic 7.62 x 39 mm round. The 6 x 49 mm round has muzzle velocity equal to the 6 x 53 mm, it is also extremely hot having higher chamber pressures than the 6 x 53 mm. While there is a risk for case separation, the case has a circular groove formed near the extractor groove to allow case stretching without risk of separation. For reliable operation, adequate headspace is critical in self-loading firearms. On the other hand, high pressure ammunition will require tight headspacing to be safe. Controlled case stretching as used with certain aircraft cannons allows use of workable headspacing, yet the gun is also safe to fire.
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