45 mm anti-aircraft gun (21-K)

45 mm anti-aircraft gun (21-K)
A 21-K on board the Krasny Kavkaz
Type	Anti-aircraft cannon
Place of origin	Soviet Union
Service history
In service	1934—?
Used by	Soviet Union
Wars	Second World War, Cold War
Production history
Designed	1932—34
Produced	1934—1947
No. built	2799
Variants	40-K, 41-K
Specifications
Weight	107–115 kilograms (236–254 lb)
Length	2.3975 metres (7.866 ft)
Barrel length	2.0725 metres (6.800 ft)
Shell	45×386 mm. SR
Shell weight	1.065–2.14 kg (2.35–4.72 lb)
Caliber	45 centimetres (18 in)
Action	single-shot
Breech	semi-automatic, vertical sliding block
Elevation	depends on the mount
Traverse	360°
Rate of fire	25-30 rpm (practical)
Muzzle velocity	880 metres per second (2,900 ft/s)
Effective firing range	6,000 m (20,000 ft) (maximum ceiling)
Maximum firing range	9,200 metres (10,100 yd) at 45°

The 45 mm anti-aircraft gun (21-K) was a Soviet design adapted from the 45 mm anti-tank gun M1932 (19-K). This was a copy of a 3.7 cm (1.5 in) German weapon designed by Rheinmetall that was sold to the Soviets before Hitler came to power in 1933 that had been enlarged to 45 mm (1.8 in) to reuse a large stock of old 47mm ammunition. It was used by the Soviet Navy to equip almost all of their ships from 1934 as its primary light anti-aircraft gun until replaced by the fully automatic 37 mm 70-K gun from 1942 to 1943. It was used in World War II and during the Cold War as the Soviets exported their World War II-era ships to their friends and allies. However it was not very effective as its slow rate of fire and lack of a time fuze required a direct hit to damage targets.

The 46-caliber21-K was a minimal adaptation of the 53-K anti-tank gun that was created by taking the latter's barrel and mounting it on a simple pedestal mount. Its semi-automatic breech automatically ejected the cartridge case and locked open, ready for the next round. This was less than ideal for an anti-aircraft weapon that relied on its rate of fire to inflict damage on aircraft because every round had to be hand-loaded. Fully automatic weapons of roughly this caliber like the 40 mm Bofors typically used 4-5 round clips of ammunition to produce rates of fire four times as high.

Early production guns had a built-up barrel, but later ones used a monobloc. There were problems with the breech mechanism early in the production run and a number of the first year's production run lacked the semi-automatic breech entirely.

The 21-K, complete with its pedestal, weighed 507 kg (1,118 lb). It was manually operated and could elevate between -10° and +85° at a rate between 10 and 20 degrees per second. It could traverse a full 360° at a rate between 10 and 18 degrees per second, although this was practically limited by its actual location on ship. In the mid-1930s special powered turrets were developed for use on river monitors. The 40-K was a single gun turret that weighed 2,000 kg (4,400 lb) and the 41-K was a twin-gun turret that weighed 2,600 kg (5,700 lb). Both turrets could elevate between -5° and +85° at a rate of 8 degrees per second and could traverse a full 360° at a rate between 4.8 and 9.8 degrees per second.