3.7 cm Flak 18

3.7 cm Flak 18/36/37/43
A 3.7 cm Flakzwilling 43
Type	Anti-aircraft cannon
Place of origin	Nazi Germany
Service history
In service	1936–45
Used by	Nazi Germany, Romania, Bulgaria
Wars	World War II
Production history
Designer	Rheinmetall
Produced	1936–45
No. built	20,243 (Germany) 360 (Romania)
Variants	BK 37 aircraft gun, 3.7 cm Flakzwilling 43
Specifications (3.7 cm Flak 43)
Weight	2,000 kg (4,400 lb) (transport)
Barrel length	3.626 m (142.8 in)
Crew	6–7
Shell	37 × 263 mm. B
Shell weight	623–659 g (1.373–1.453 lb)
Caliber	37 mm (1.5 in)
Barrels	57 calibers
Breech	gas-operated bolt
Carriage	three-legged platform
Elevation	-7° 30' to +90°
Traverse	360°
Rate of fire	150 rpm (practical), 250 rpm (theoretical)
Muzzle velocity	770–820 m/s (2,500–2,700 ft/s)
Effective firing range	4,200 m (4,600 yd) (anti-aircraft)
Maximum firing range	7,995 m (8,743 yd) (ground range)
Feed system	8-round clips

The 3.7 cm Flak 18/36/37/43 was a series of anti-aircraft cannon produced by Nazi Germany that saw widespread service in the Second World War. The cannon was fully automatic and effective against aircraft flying at altitudes up to 4,200 m. The cannon was produced in both towed and self-propelled versions. Having a flexible doctrine, the Germans used their anti-aircraft pieces in ground support roles as well; 37 mm caliber guns were no exception to that. With Germany's defeat, production ceased and, overall, 37 mm caliber anti-aircraft cannon fell into gradual disuse, being replaced by the Bofors 40 mm gun and later, by 35-mm anti-aircraft pieces produced in Switzerland.

The original 37 mm gun was developed by Rheinmetall in 1935 as the 3.7 cm Flak 18. It had a barrel length of 57 calibers (hence the additional designation L/57), which allowed 4,800 m (15,700 ft) maximum ceiling. The armour penetration was considerable when using dedicated ammunition, at 100 m distance it could penetrate 36 mm of a 60°-sloped armour, and at 800 m distance correspondingly 24 mm. It used a mechanical bolt for automatic fire, featuring a practical rate of fire of about 80 rounds per minute (rpm). The gun, when emplaced for combat, weighed 1,750 kg (3,860 lb), and complete for transport, including the wheeled mount, 3,560 kg (7,850 lb).

The Flak 18 was only produced in small numbers, and production had already ended in 1936. Development continued, focusing on replacement of the existing cumbersome dual-axle mount with a lighter single-axle one, resulting in a 3.7 cm Flak 36 that cut the complete weight to 1,550 kg (3,420 lb) in combat and 2,400 kg (5,300 lb) in transport. The gun's ballistic characteristics were not changed, although the practical rate of fire was raised to 120 rpm (250 rpm theoretical). A new, simplified sighting system introduced the next year produced the otherwise-identical 3.7 cm Flak 37. The Flak 37 was known as 37 ITK 37 in Finland.

The Flak 36/37 were the most-produced variants of the weapon.

In 1938, the Kingdom of Romania acquired the license to locally produce 360 guns, officially known as "Tun antiaerian Rheinmetall calibru 37 mm model 1939" ("37 mm Rheinmetall anti-aircraft cannon model 1939") at the Astra factory in Brașov. By May 1941, 102 had been built, and by October 1942, the production rate was 6 pieces per month.