45 mm anti-aircraft gun (21-K)

The 45 mm anti-aircraft gun (21-K) was a Soviet design adapted from the 45 mm anti-tank gun M1937 (53-K). This was a copy of a 3.7 cm 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 in increase its penetrating power. 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.

Design
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.

Description
The 21-K, complete with its pedestal, weighed 507 kg. 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 to 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 2000 kg and the 41-K was a twin-gun turret that weighed 2600 kg. 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 to 9.8 degrees per second.

Ammunition
The 21-K used the same ammunition as the 45 mm anti-tank gun M1937 (53-K). The biggest problem in its role as an anti-aircraft gun was that it wasn't provided with a time fuze that would detonate the shell at a specified distance from the gun. This meant that only a direct hit would damage its target.