37 mm automatic air defense gun M1939 (61-K) () was a Soviet 37 mm caliber anti-aircraft gun developed during the late 1930s and used during World War II. The land based version was replaced in Soviet service by the ZSU-57-2 during the 1950s. Guns of this type were successfully used throughout the Eastern Front against dive bombers and other low- and medium-altitude targets. It also had some usefulness against lightly armored ground targets. Crews of the 37 mm AD guns shot down 14,657 Axis planes. The mean quantity of 37 mm ammunition to shoot down one enemy plane was 905 rounds.