3 . 7 cm Flak 18 / 36 / 37 / 43 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 , 35 - mm anti - aircraft pieces produced by Switzerland . Development . 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 effective 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 , and complete for transport , including the wheeled mount , . 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 in combat and 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 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 . Flak 43 . The 3 . 7 cm Flak 43 was a dramatic improvement over older models . A new gas - operated breech increased the practical firing rate to 150 RPM , while at the same time dropping in weight to in combat , and in transport . It was also produced in a twin - gun mount , the 3 . 7 cm Flakzwilling 43 , although this version was considered somewhat unwieldy and top - heavy . Many Flak 37s were mounted on the ubiquitous Sd . Kfz . 7 half - track vehicle , or later the " schwere Wehrmachtschlepper " ( " sWS " ) , but the newer Flak 43 was almost always used in a mobile mounting . Most famous of these were the converted Panzer IV s , first the " interim " Möbelwagen , and later the Ostwind , which was considered particularly deadly . Compared to its closest Allied counterpart , the 40 mm Bofors L / 60 , the Flak 43 had twice the rate of fire and was both notably lighter and more compact ; the Bofors was slightly more powerful ( with greater range , ceiling , and a shorter projectile flight time ) and fired a more destructive shell . Large - scale production did not start until 1944 , and about 7 , 216 had been produced by the end of the war ( " Zwillings " included , each counted as two guns ) . 3 . 7 cm Flak 43 M43U . The 3 . 7 cm Flak 43 M43U was the marine version of the 3 . 7 cm Flak 43 used by the " Kriegsmarine " on Type VII and Type IX U - boats . It was mounted on the LM42U mount .