Sten

The Sten was built for one reason: the British needed a cheap, easy to manufacture submachine gun that could interchange with captured German MP40 ammunition. It did this well enough, although many British soldiers preferred the German Shmisser.

WWII
The Sten was an all important weapon for the British war effort, and the weapon was produced by the millions for British troops and for resistance movements under German occupation. It was easy to hide from Gestapo searches and could easily be used by paratroopers. Despite it's being a weapon born of desperation,it still faithfully served the British through WWII.

Post War Use
The Sten was used extensively in Israel's war for independence, and was used by the Royal Navy in the Korean War. Suppressed versions were used in the Vietnam War by the US Special Forces, and the prime minister of India was killed with one in 1984.