The T-21 was the heaviest standard-issue weapon that could be carried by a single soldier. Clone troopers of the Grand Army of the Republic carried T-21s into battle during the later stages of the Clone Wars, and the weapon was carried over in the transition from Galactic Republic to the Galactic Empire. Due to its portability, it was also popular with the Rebel Alliance, and later the New Republic.
Since the T-21's standard power pack only has enough energy for twenty-five shots, it was almost always paired with a continuous-feed power generator. The twenty-kilogram generator was carried as a backpack.
A First World War-era Lewis machine gun provided the basis for the T-21. The Lewis gun was the standard light machine gun of British and Commonwealth forces during World War I. It was fed by a drum magazine that was placed horizontally on top of the gun's receiver. To produce the T-21 prop, the drum magazine was removed and minor cosmetic alterations were made.