noun
heavy machine gun
A large-caliber, fully automatic firearm designed for sustained fire, typically mounted on a tripod or vehicle.
敵の陣地に重機関銃が設置されていた。
A heavy machine gun was set up in the enemy position.
軽機関銃 is a light machine gun, which is more portable and often used by infantry, while 重機関銃 is heavier and designed for sustained fire from a fixed position.
Compound of 重 (heavy) + 機関銃 (machine gun). The term is a straightforward calque from English 'heavy machine gun'.