The Nkm wz.38 FK was a 20 mm heavy machine gun produced in inter-war Poland. It was used both in anti-air and anti-tank role and was also adapted for tank use and mounted on some TKS tankettes. Mass-produced in 1938 and 1939, it was used extensively during the Polish Defensive War of 1939.

20 mm caliber is a specific size of popular autocannon ammunition. The dividing line between smaller-caliber weapons, commonly called "guns", from larger-caliber "cannons", is conventionally taken to be the 20 mm round, the smallest caliber of autocannon. All 20 mm cartridges have an outside projectile (bullet) diameter and barrel bore diameter of approximately 0.787 inches (20.0 mm). These projectiles are typically 75 to 127 mm (3–5 in) long, cartridge cases are typically 75 to 152 mm (3–6 in) long, and most are shells, with an explosive payload and detonating fuze.