The original
PK was a development of
Kalashnikov's
AKM assault rifle and the accompanying
RPK light machine gun design that featured stamped receivers. The PK uses the
7.62×54mmR Eastern Bloc standard cartridge that produces significantly more
bolt thrust when compared to the Eastern Bloc
7.62×39mm and
5.45×39mm intermediate cartridges. With the use of a single spare parts kit and two barrel assemblies the service life of the modernised PKM machine gun series is guaranteed for at least 25,000 rounds.
Operating mechanism The bolt and carrier design are similar to the
AK-47 and other modernised
Kalashnikov-pattern designs, as is the stripping procedure performed to remove those mechanisms from the gun for cleaning. The bolt and bolt carrier are however oriented upside down compared to the AKM, with the piston and gas system being underneath the barrel. Unlike the AKM and RPK the PK machine gun series is an open bolt design, which improves heat management during automatic fire compared to closed bolt designs and helps avoiding the dangerous phenomenon known as "
cook-off", wherein the firing chamber becomes so hot that the propellant contained in a chambered round unintentionally ignites, making the machine gun fire until the ammunition is exhausted. Open bolt designs typically operate much cooler than closed bolt designs due to the airflow allowed into the chamber, action and barrel during pauses between bursts, making them more suitable for constant fully automatic fire. General-purpose machine guns like the PK are further normally issued with several quick change barrels that during prolonged intense use are swapped out allowing one barrel to cool while the machine gun fires with the other. The rimmed 7.62×54mmR cartridges are set in a metal ammunition belt and are held against the shoulder inside non disintegrating looped links, leaving the rim exposed at the rear. The belt is mounted from the right side into the feed way of the PK machine gun. The
machine gun uses a non-reciprocating charging handle on the right side of the receiver to charge the gun. Since the PK uses a rimmed rifle cartridge and closed-link belts used for feeding, a two-stage feed mechanism with a preliminary extraction of a cartridge from a belt link was preferred over a direct ammunition feed design often used for rimless cartridges. The PK machine gun is equipped with a lever-type feed mechanism introduced in Louis Stange's MG 39 Rh and copied in the Czechoslovak machine guns like
vz. 52 and
vz. 59, which is operated by the feed lever. The lever, which is simplified compared to the prior art, is mounted on the right wall of the receiver and wraps around the bolt carrier with its feed pawl and roller. The rest of the mechanism is mounted either on the receiver cover or on the ammunition feed tray cover hinged under it. The PK feed mechanism pulls the
rimmed 7.62×54mmR cartridges out from the back of the ammunition belt and drops the cartridges down into the feed way, allowing the bolt to strip and feed the cartridges into the chamber for firing. The PK feed mechanism is radically different from that of
7.62×51mm NATO machine guns based on the
MG 42 feed mechanism that typically incorporate a much larger (and therefore much heavier) articulated feed cam, lever, and pawl assembly that pushes
rimless cartridges out forward from their links directly into the chamber for firing. The PK fires from the rear sear. The breech is locked by a rotating bolt, with two locking lugs engaging locking recesses in the receiver. The gas piston is hinged to the bolt carrier assembly, and its vertical travel makes it possible to bend the group making machine gun assembly and disassembly for maintenance easier. The protruding rear part of the bolt carrier assembly features spiral shaped cuts, which provide a controlled rotation of the bolt. The mainspring is accommodated in the bolt carrier assembly slide channel. A cartridge extractor with a latch is mounted in the rear part of the bolt carrier assembly. The cocking lever, mounted on the right, is not integral with the bolt carrier and does not reciprocate as the gun fires. The gas cylinder is mounted under the barrel and fitted with a gas regulator with three fixed positions. The gas regulator opens corresponding holes to change the amount of expanding propellant gases bled off out of the gas cylinder into the atmosphere, thus varying the amount of energy transferred on to the long-stroke piston.
Receiver The PK general-purpose machine gun U-shaped
receiver is stamped from a smooth
sheet of
steel that is supported extensively by pins and rivets. For additional rigidity and strength the PK receiver features double walls made from 1.5 mm steel plates that are welded together with the U-shaped stamping. The receiver top cover is also stamped from 1.5 mm sheet metal and hinged on the front of the receiver and locked at the back with a spring-loaded latch.
Barrel The quick detachable barrel assembly slides into the receiver and attaches by a barrel-lock. On the original PK it was partially
fluted to increase rigidity and improve heat dissipation. The barrel-lock also regulates the gap between the breech face and the breech end of the barrel. PK barrels have a folding carry handle/grip that is positioned to the left of the receiver and is used to transport the machine gun and quickly and safely change-out barrels to prevent barrel overheating. The bore is
chrome-lined and features four right-hand grooves at a 240 mm (1 in 9.45 in) rifling twist rate. The muzzle is threaded for the installation of various muzzle devices such as a
flash hider. The muzzle was normally equipped with a conical flash suppressor that added to the barrel and later a long slotted flash suppressor that added to the barrel. Later when the PKM variant was introduced the PK series barrel fluting was omitted and the muzzle device was changed to a shorter cylindrical slotted flash suppressor that added to the barrel. The PKM barrel assembly weighs and can fire up to 400 rounds in
rapid fire scenarios before it has to be replaced for another barrel or allowed to cool down to prevent unacceptable wear of the bore. The sustainable
effective rate of fire is about 250 rounds per minute. Whilst, the cyclic rate of fire is around 600–800 rounds per minute. A later designed similar optical sight suitable for the PK machine gun series is the 4×24 1Р77.
Trigger The trigger assembly, mounted inside the receiver, is operated by the mainspring and suitable for automatic fire. It has no single shot mode of fire. The manual rotating type safety locks the sear, which engages the sear notch of the bolt carrier assembly, and the trigger lug does not allow the bolt carrier assembly to go all the way back.
Stock The skeletonised buttstock, pistol grip and folding carry handle/grip on the barrels were originally manufactured from
birch plywood laminates. Such engineered woods are stronger and resist warping better than the conventional one-piece patterns, do not require lengthy maturing, and are cheaper. The wooden furniture was finished with the Russian amber shellac finishing process. Small accessories and an oil-solvent container can be stored inside butt recesses. Later the buttstock was fitted with a hinged butt-rest. More recent PKM machine guns and barrel assemblies are equipped with a new black glass-filled polyamide buttstock, pistol grip and barrel carry handle/grip shaped like the previously used laminated wooden stock and grips.
Feeding used by the PK machine gun PK machine guns are belt-fed, using non-disintegrating metal
belts, which have links that wrap around the cartridge case shoulder all the way around, and are linked by a coiling wire on each side. The links are made of thick high carbon stamped steel sheet metal that is zinc phosphated and varnished for protection. These belts are preloaded at ammunition factories in 25-round connectable belt lengths and can be linked to any length necessary. Factory connected PK ammunition belts are available in 25, 100, 200 and 250 rounds lengths. Typical of Soviet machine guns, the PK feeds from the right and ejects its spent cases via an ejection port on the left side of the machine gun, contrary to the right side ejection port seen in most Western machine guns. For the
light machine gun role, the PK is used as the standard squad automatic weapon of the Russian Army. The PK uses a 100-round non-disintegrating belt contained in a metal box made from an aluminium frame and steel cover that can be attached under the gun's receiver. The 100-round belt "assault" box has a folding lid in its cover for feeding the ammunition belt when the box is attached under the machine gun receiver and weighs or for the modernised all aluminium lightweight variant. When the machine gun is fired from a bipod, the 100-round ammunition box is normally attached to the underside of the receiver. For the
medium machine gun role, there is also a 200/250-round ammunition box made from an aluminium frame and steel cover available which can be mounted on the tripods used for the PK machine gun series. A 200/250-round ammunition box containing a 250-round non-disintegrating belt weighs and containing a 200-round non-disintegrating belt weighs or for the modernised all aluminium lightweight variant. Both metal ammunition boxes have canvas carry handles. All openings on the machine gun, particularly the ejector port on the left and the belt feed entrance on the right, are covered with spring-loaded dust covers so that the openings are only exposed when they need to be.
Accessories The PK is equipped with a simple detachable bipod mounted to the gas cylinder beneath the barrel and in that light machine gun setup is used as a squad-level support weapon. The right bipod leg accommodates links of a cleaning rod. Other accessories include a sling and storage covers. The PK machine gun is also suitable for installation on tripod mounts or vehicle mounted medium machine gun setups and can also be used as a light
anti-aircraft weapon against slow flying aircraft when it is put on an AA mount. As with all general-purpose machine guns, tripod and vehicle mountings offer a higher degree of accuracy and control than when used on a less stable bipod. The PK machine gun, firing short bursts from a bipod, as a light machine gun has the following accuracy of fire: a mean deviation of at a range of , at , and at . The Russian and other European militaries use a
circular error probable method that assumes a 50% hit probability (R50). ==Variants==