Nkm wz.38 FK

The Nkm wz.38 FK (Polish military acronym meaning Heaviest Machine Gun Mark 1938) was a 20mm heavy machine gun (autocannon by modern terminology) produced in inter-war Poland. It was used both in anti-air and anti-tank role, it was also adapted to tank use and mounted on some TKS tankettes. Mass-produced in 1938 and 1939, it was used extensively during the September Campaign.

Name
In line with other arms design of the epoch, the Nkm wz.38 FK was a code-name rather than a proper name of the weapon. The Nkm acronym stands for najcięższy karabin maszynowy, literally the heaviest machine gun (i.e. heavier than standard contemporary heavy machine guns using standard rifle cartridges such as the 7.92 Mauser, .303 British or 7.62×54mmR). The second part of the name, '''wz. 38''' stands for wzór 38, that is Mark 1938, referring to the year the weapon was submitted to the military for testing or the year the weapon was designed. Finally, when more than one weapon of certain type was conceived in the same year, additional designation was added at the end to differentiate between various types. In this case the FK stood for Fabryka Karabinów, the Warsaw-based state-owned rifle factory.

Origins
In early 1930s the Polish Army was looking for a modern anti-tank and anti-air weapon to replace outdated World War I equipment still in use. In 1931 heavy machine guns made by Hotchkiss, Solothurn and Oerlikon were tested, but were found unsuitable for Polish needs. Most importantly, they could not suit both AT and AA roles well. In 1937 another commission was sent abroad to test the newly designed weapons of 20mm calibre by Oerlikon, Madsen and Hispano-Suiza. As all designs had several flaws, it was decided to start works on an indigenous design. Engineer Wawrzyniec Lewandowski of Warsaw-based Fabryka Karabinów company became the lead designer.

The prototype, dubbed nkm model A, was built already in November 1937. It was tested and by March 1938 most design flaws were eliminated. During additional tests it showed excellent penetration capabilities: it could penetrate 40mm of steel armour at 200 metres distance. It was found better than the competing foreign products as an anti-tank weapon and anti-air gun. However, usage as aircraft weapon was discouraged due to low firing rate and the new weapon was found slightly too heavy for infantry and cavalry use. Because the test results were satisfactory, in May of that year the Polish Army ordered additional two copies for additional tests.

As an anti-tank weapon for infantry use the gun was to be mounted on two types of mountings: light (a simple tripod) and heavy (modified mounting used by 37mm Bofors AT gun). With time the Polskie Zakłady Optyczne works also designed new anti-air sights (similar to those used in 40mm Bofors AA gun) and the gun was finally accepted for service under the official designation nkm wz.38 FK, the FK being the acronym of the lead design bureau at Fabryka Karabinów.

A first batch of 100 HMGs was ordered on August 26, 1938. The gun was produced by Sanok-based Zieleniewski company, with barrels made by Pruszków-based Zakłady Przemysłowe Stowarzyszenia Mechaników Polskich z Ameryki works and newly designed ammunition at the State Munition Works in Skarżysko-Kamienna. In April 1939 another 140 pieces were ordered, 40 of those as stationary weapons (without mountings) and with time the production was to reach 100 pieces a month.

However, the initial costs were high. The design itself, factory equipment and the guns of the first batch themselves cost roughly 2.2 million złoty. The plans for budget year 1939/1940 amounted to almost 3.4 million. In addition, the initial batch was being manufactured at a much slower rate than expected. Because of that by the outbreak of World War II out of 896 pieces ordered for delivery by 1940 only 55 pieces were delivered to the Polish Army.

Usage
The Nkm wz.38 FK was to become a standard anti-tank and anti-air equipment of the Polish Army. As a tank gun it was to be mounted on PZInż. 130 amphibious tanks, as well as 4TP light tanks. In addition, as a stop-gap solution roughly 100 TK-3 and TKS tankettes were to be rearmed with this HMG. As an anti-tank and anti-air weapon, the gun was to be used extensively: 8 pieces in AA configuration for every infantry division. Also all anti-tank companies were to be eventually reequipped with this gun.

However, as by September 1, 1939 the overall production did not exceed 55 pieces, only 40 were mounted on TK-3 and TKS tankettes. The fate of the 15 pieces on infantry mountings remains unknown.

Operational history
The armoured units to be equipped with the new 20mm weapon needed a new gun mounting. In mid-1938 TK-3 and TKS tankettes rearmed with the new gun were tested extensively. The new vehicle was to be used primarily in a tank destroyer role. In January 1939 the new TKS tankette armed with 20mm HMG was accepted into service. A major drawback of this stop-gap solution was low load of a tankette – it could transport only a limited supply of ammunition. However, it also offered high mobility and very low profile.

Description
The Nkm wz.38 FK feature blowback operation. The barrel was fixed with a muzzle brake with a flame suppressor. The weapon was fed either with small box magazines for 5 or 10 rounds, or drum magazines for 15 or 100 rounds.

The specially-designed 20mm ammunition was based on 20x138mm Solothurn long shell. It was produced in several configurations: armour-piercing tracer, incendiary tracer, as well as training round and training tracer round. The set was later extended to include also AA ammunition, HE tracer.

Penetration Capacity

 * Penetration Capacity:

Note: Capable of destroying any German tank of 1939 except for possibly Panzer IV (The Panzer IV was used in Poland in field test numbers; yet, some got destroyed) and Czechoslovak—made Panzer 38(t). The more recently provided photos show that the tank driven by Prince Wiktor IV Albrecht von Ratibor, destroyed by Mr. Cadet Roman Orlik, was indeed a Panzer IV and not a Panzer 35(t).