The HK21 is a German 7.62 mm general-purpose machine gun, developed in 1961 by small arms manufacturer Heckler & Koch and based on the G3 battle rifle.

The HK21 is a selective fire roller-delayed blowback-operated firearm with a semi-rigid locking mechanism designed to retard the rearward movement of the bolt.

During the "unlocking" sequence, the bolt head receives the recoil impulse from the ignited cartridge and exerts rearward pressure against the rollers, seated in recesses in the barrel extension.

The reliable functioning of roller-delayed blowback mechanisms is limited by specific ammunition and arm parameters like bullet weight, propellant charge, barrel length and amount of wear.

For obtaining a proper and safe functioning parameters bandwidth Heckler & Koch offer a variety of locking pieces with different mass and shoulder angles.

The angles are critical and determine the unlock timing and pressure curve progression as the locking pieces act in unison with the bolt head carrier.

The weapon has a hammer striker and is fired from the closed bolt position, rare for general purpose machine guns.

By simply swapping out several components such as the barrel, bolt and feed plate unit, the weapon can be quickly converted to the intermediate 7.62×39mm and 5.56×45mm NATO chamberings.

The GR-series were "sanitized" (i.e., having no serial numbers or identifying marks) Heckler & Koch weapons used by special operations forces.

The machine gun's weight was increased, a carrying handle and a hooked buttstock with improved buffer mechanism were also added.

During the 1980s both the HK21A1 and HK11A1 were modernized based on user and testing feedback, resulting in a new modular family of machine guns that share the same receiver, trigger group and interchangeable barrels and feed units, consisting of: The "E" simply stands for "Export" model.

Compared to the older HK21A1 and HK11A1, the modernized "Export" weapons also feature a longer barrel shroud, extended by 94 mm (3.7 in) towards the front, which also resulted in a longer sight radius; the barrel extension was modified (the HK21E also received a longer barrel); a burst fire mode was incorporated into the trigger group as the fourth selector setting (3-round burst); a polymer barrel handle was added; space for a cleaning kit was added in the grip; the rear sight was modified with an adjustable drum; a bipod with a 3-position height adjustment and 30-degree tilt mechanisms replaced the simple folding bipod; the feed mechanism was modified to provide continuous feeding (by moving the belt in two stages, during both forward and rear movement of the bolt), a forward assist for silent loading was provided and the machine guns were adapted to use NATO-standard optical sight mounts.

When employed in the light machine gun (infantry assault) role, the HK21E stores its ammunition belt in a 100-round sheet metal container, fastened to the bottom of the feed mechanism (as in the HK21A1).