The HK21 is a light machine gun powered belt of ammunition, firing rounds of caliber 7.62mm NATO. Designed by German arms manufacturer Heckler & Koch. The weapon can be fired in a semi-automatic or automatic. The HK21 was developed from the automatic rifle G3. Sharing this with several components, including the firing mechanism and backflow. The HK21 can be used with its integral bipod or a tripod. It was also license-manufactured by Fábrica do Braço de Prata in Portugal as the m/968 and in Mexico by SEDENA as the MG21. In the German military (Bundeswehr) and the federal police (Bundespolizei) it is designated “G8”.
The Heckler & Koch 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. This delay was achieved by artificially increasing the inertia of the bolt by using an angular, interposed transmission system, installed symmetrically to the bore axis, with two cylindrical rollers acting as transmission elements against a movable locking piece which drives the heavy bolt carrier.
The two-piece bolt assembly consists of a bolt head, which contains the aforementioned rollers, and a supporting locking piece and bolt carrier. 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 rollers are driven inward against angled ramps of the barrel extension and interact with the wedge-shaped locking piece, projecting it backwards.
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Thus a 4:1 transmission ratio is maintained of the bolt carrier and locking piece relative to the bolt head; the bolt carrier travels backwards four times faster than the bolt head, ensuring a safe drop of pressure within the barrel prior to extraction. Since extraction is carried-out under relatively high pressure, the barrel’s chamber received a series of flutes designed to help free the bloated cartridge casing from the chamber walls.
The bolt features a spring-powered extractor and an anti-bounce device that prevents the bolt head from glancing off the barrel extension upon forward return of the locking assembly. The lever-type ejector system is contained in the trigger group housing and is actuated with every shot by the recoiling bolt. The weapon has a hammer striker and is fired from the closed bolt position. The trigger group, which is integrated with the pistol grip and hinged from the receiver, is equipped with a fire control selector switch (selector lever in the “E” or “1” position – semi-automatic fire, “F” or “20” – continuous fire mode) that doubles as a manual safety (rotating the lever into the “S” or “0” setting disables the trigger, weapon is considered “safe”).
The machine gun feeds from the left-hand side through a variety of disintegrating ammunition belt types: the American M13 linked belt, the German DM6 counterpart to the M13 or the segmented German DM1 belt. The ratcheting wheel feed unit was designed as an easily removable module that is inserted into the bottom portion of the feed block, below the barrel axis. As a result of this configuration, the ammunition belt is aligned upside down compared to most other belt-fed weapons.
The feed mechanism is actuated by the reciprocating movement of the bolt; a curved cam slot on the bottom of the bolt engages an actuator in the feed mechanism, rotating the double sprockets and positioning a new round in the feed path. Simple conversion from belt to magazine feed is possible by installing an adapter in the feed block which enables the use of H&K’s proprietary 20-round box magazine (from the G3 rifle) or a 50-round drum magazine. The HK21 uses a modified G3 receiver that has been extended to the front sight base and is equipped with a detachable bipod (mounted either in front of the feed mechanism or at the muzzle) and tripod and vehicle mounting points. The HK21 has close to a 48% parts interchangeability with the G3.
The HK21 has a heavy, quick-change barrel fitted with a slotted flash suppressor and adjustable iron sights with a hooded foresight and an aperture rear sight adjustable from 100 through 1200 m in 100 m increments. By simply swapping out several components such as the barrel, bolt and feed unit, the weapon can be quickly converted to the intermediate 7.62x39mm and 5.56x45mm calibers.
G8 and G8A1
The German Army, German Navy and the Federal Police use a variant of the HK11 designated the Gewehr-8
(or G8). It is tapped for telescopic sights and has a quick-change barrel with a bipod that uses either a heavy match-grade bull-barrel or a heavy barrel for automatic fire. It was designed to use G3-type 10- or 20-round box magazines in the designated marksman role, but could also use a special 50-round drum magazine for sustained supporting or suppressing fire. The modified G8A1 adopted the improvements of the HK11A1 series and was only able to feed from magazines and drums.
The GR-series were “sanitized” (i.e., having no serial numbers or identifying marks) Heckler & Koch weapons used by special operations forces. They differed from the stock weapons in that they had optical sights, no provisions for iron sights, and came standard in Woodland (-C suffix) or Desert (-S suffix) camouflage.
- GR-6 Automatic Rifle (HK13).
- GR-9 Light Machinegun (HK23).
In the early 1970s the HK21’s design was simplified and the feed mechanism was modified. The machine gun’s weight was increased, a carrying handle and a hooked buttstock with improved buffer mechanism were also added. Since then, the HK21 was offered in two main variants: the HK21A1 general purpose machine gun and the HK11A1 automatic rifle (optimized for magazine feed only). The HK11A1 proved more successful in export sales and was adopted by the Hellenic Army and several other African and Asian armies. Both weapons, as in the original HK21, can be converted to chamber the 5.56x45mm NATO round by replacing the appropriate components.
During the 1980s both the HK21A1 and HK11A1 were modernized, resulting in a new modular family of machine guns that share the same receiver, trigger group and interchangeable barrels and feed units, consisting of:
- HK11E automatic rifle (magazine fed, chambered in 7.62x51mm NATO caliber)
- HK13E automatic rifle (magazine fed, chambered in 5.56×45 mm NATO caliber)
- HK21E general purpose machine gun (belt feed, 7.62x51mm NATO), which is also license-built in Mexico for the armed forces
- HK23E light machine gun (belt-fed, 5.56x45mm NATO).
The “E” simply stands for “Export” model.
Compared to the older HK21A1 and HK11A1, the modernized “Export” weapons feature a longer receiver, extended by 94 mm (3.7 in) towards the front, which also resulted in a longer sight radius; the barrel trunion 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; the rear sight was modified with an adjustable drum; a bipod with a 3-position height adjustment mechanism 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 useNATO-standard optical sight mounts. When employed in the 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). The ability to use G3 or STANAG (for HK23E) box or drum magazines is available with an optional feed module kit that consists of the bolt, recoil spring and belt feed module or magazine well. The shock-absorbing tripod weighs 10.5 kg (23 lb).