# 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 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.

## 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 (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.

### 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.
| Shell name | Type | Weight | Filling Weight | Muzzle velocity |
| BR-240 | armor-piercing | 1.42 kg (3.1 lb) | 18 g (0.63 oz) | 760 m/s (2,500 ft/s) |
| OT-033 | Fragmentation-tracer | 1.065 kg (2.35 lb) | 52 g (1.8 oz) | 880 m/s (2,900 ft/s) |
| OR-73A | Fragmentation-tracer | 1.41 kg (3.1 lb) | 37 g (1.3 oz) | 760 m/s (2,500 ft/s) |
| F-73 | High-explosive | 1.41 kg (3.1 lb) | 74 g (2.6 oz) | 760 m/s (2,500 ft/s) |
| O-240 | High-explosive | 2.14 kg (4.7 lb) | 118 g (4.2 oz) | 335 m/s (1,100 ft/s) |