The PU series of light machine guns were developed between 1971 and 1978 at the IZHMASH arms factory in Izhevsk, Russia. Design team for all variants included Juri Alexandrov, Victor Kalashnikov (son of Mikhail Kalashnikov) and Mikhail Dragunov (son of Evgenij Dragunov). Development work was performed under the “Poplin” R&D program, sponsored by the Soviet Army. This program sought a new 5.45mm light machine gun, 150-200% more efficient than the 7.62mm RPK light machine gun then in service.
The 5.45mm PU light machine gun was based on the experimental magazine-fed P-3 light machine gun, which later evolved into the RPK-74, designed by the team led by Mikhail Kalashnikov. The PU featured the already classic Kalashnikov RPK-style design with reinforced receiver and longer barrel. The belt feed unit was mounted on the left receiver wall. Non-disintegrating steel belt with open pockets was designed specifically for this gun. Feed direction was from bottom to top, and belt feed unit had a side-opening cover. To ensure sufficient reliability when feeding the long belt, gun was equipped with a manual gas regulator. Box magazines were inserted in the usual way, from below. When used with the belt, aluminum belt boxes were clipped to the gun using the empty magazine housing. Gun also retained AK-style hammer-fired trigger unit with three position safety / fire mode selector. The PU light machine gun fired in full- and semi-automatic modes, from a closed bolt. It was tested in 1975, and proved to be insufficiently durable and reliable. As a result, a slightly improved version, the PU-1 machine gun, was built and tested in 1976. But it still was not good enough.
The 5.45mm PU-2 light machine gun was designed in 1977 from the ground up, and shared very little with the RPK-74. It used a rotary bolt locking, but the gas system with long stroke piston and manual gas regulator was located below the barrel. Belt feed unit was placed above the receiver, and magazine housing was located on the left side of the gun, with magazine sticking out horizontally. Magazine housing was provided with a hinged dust cover. The PU-2 light machine gun fired from the open bolt and only in full automatic. However, it still had a non-detachable barrel. For some unknown reason the PU-2 machine gun had an overly complex belt feed unit, which performed unsatisfactory during the tests.
The 5.45mm PU-21 light machine gun, which was designed and built in 1978, was the last in the family of dual fed 5.45mm PU machine guns, tested under the “Poplin” R&D program. It was more or less similar to the PU-2 in its overall layout, but featured simplified belt feed, and a new bipod mount with the hinge located above the barrel for improved stability. Gun performed more or less satisfactory during the early tests. However, by late 1978 Soviet Military lost its interest in the program, apparently being satisfied with the new magazine-fed RPK-74 machine gun, which by that time was already entering service in significant numbers. As such, no further development of dual-fed (belt and magazine) light machine guns was undertaken in USSR, despite the international success of the 5.56mm FN Minimi light machine gun, which appeared at the roughly same time. It also must be noted that the first production light machine gun to feature easily switchable dual belt / magazine feed was the LK Vz.52 light machine gun, designed in Czechoslovakia and adopted in 1952.
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PU-21 light machine gun, basic specifications: