In 1949, Heckler & Koch GmbH was formed at Oberndorf am Neckar in West Germany to become a small arms weapons designer and manufacturer. Today, HK is one of the world's finest small arms producers whose product line has since included battle rifles, assault rifles, submachine guns, personal defense weapons, designated marksman rifles, sniper rifles and various air-cooled machine guns. HK saw an opportunity in 1961 to branch out and design a new "universal" machine gun and this development became the "HK21" series. The HK 21 was formally categorized a as "general purpose machine gun" (GPMG) - though it is also considered a "light machine gun" (LMG) - and went on to spawn of family of support weapon. The family was based on the hugely successful HK G3 Battle Rifle / Assault Rifle which went on to see widespread use around the world. The original G3 automatic rifle design utilized a 20-round straight detachable box magazine as well as a 50-round drum for supporting fire. Since the HK21 originated from the G3, the HK21 utilized a similar receiver, could operate from a ammunition magazine or belt feed and incorporated a detachable bipod for stabilization in the fire support role.
The HK21 was initially designed as a belt-fed weapon however an adapter was then produced to allow the use of box magazines or an ammunition drum. All Heckler & Koch machine guns (including the HK21) feature the same successful roller-delayed blowback action - based on the original late-World War 2-era StG45 prototype - with a fixed barrel. A two-part breech block and roller delays were standard and a smooth extraction was due to a fluted chamber. The firing action was sequenced by a two-position lever to facilitate either single, semi-automatic fire or for full-automatic fire. If the operator wanted to fire the 7.62x39mm or the 5.56x45mm standard cartridge a simple change of barrel, feed plate and the bolt would suffice in creating an all-new weapon platform. To prevent the bolt from striking obliquely against the barrel and return the lock assembly, a spring-powered extractor was used. The recoiling bolt utilized energy from the preceding shot and would extract the cartridge via this lever ejector system. The selector switch was labeled clearly with an "E" for semi-automatic fire, an "F" for full-automatic fire and rotating the lever to the "0" or "S" setting would block firing to enter a "safety" mode.
To accept a number of multiple ammunition feed systems HK also devised a module to allow use of the German DM6 and DM1 belt as well as the American M13 belt. This module was fitted below the weapon in which the bolt would pass over the ammunition belt - set upside down so the links face down. The new round was then fed into position by the reciprocating action of the bolt. A heavy barrel with a flash suppressor was standard as were adjustable iron sights, this consisting of a forward and rear sight with the rear sight ranging from 100m to 1200m in 100m increments. The HK21 was given a fixed, though adjustable and removable, polymer stock. A collapsible carrying handle located at the center mass of the weapon was provided for ease of transportation of the weapon in whole. In addition to a bipod, the HK 21 could be affixed to a tripod or vehicle pintle mount.
After some time in the field, the HK21 line was simplified and improved to become the HK21A1 in the 1970s. The HK11 series was similar to the HK21 though differentiated by its magazine-only feed and classification as an automatic rifle. The HK21E was an export model chambered for the 7.62x51mm NATO cartridge. The HK23E was an export light machine gun chambered for the 5.56x45mm NATO cartridge. The HK13E was another automatic rifle development and fed by magazines holding 5.56x45mm NATO cartridges. The HK G8 offered telescopic sights, a quick-change barrel and various box magazine and one drum magazine ammunition count. The HK GR-6 and GR-9 were issued to special forces units and had their serial identifiers removed. The GR-6 represented the automatic rifle form while the GR-9 was the light machine gun form.
Portugal license-produced the HK21 as the m/986 while Mexico designated their local variants (HK21E) as the MG21 with production by SEDENA.