Heckler & Koch HK21

HK21
HK21A1 general-purpose machine gun
Type	General-purpose machine gun
Place of origin	West Germany
Service history
In service	1961–present
Used by	See Users
Production history
Designer	Heckler & Koch
Designed	1961
Manufacturer	Heckler & Koch Fábrica de Braço de Prata SEDENA
Produced	1961–present
Variants	See Variants
Specifications
Weight	HK21: 7.92 kg (17.46 lb) HK21A1: 8.30 kg (18.3 lb) HK11A1: 7.70 kg (17.0 lb) HK21E: 9.30 kg (20.5 lb) HK11E: 8.15 kg (18.0 lb) HK23E: 8.75 kg (19.3 lb) HK13E: 8.00 kg (17.64 lb) HK25: 16.30 kg (35.9 lb)
Length	HK21: 1,021 mm (40.2 in) HK21A1, HK11A1, HK11E, HK23E, HK13E: 1,030 mm (40.6 in) HK21E: 1,140 mm (44.9 in) HK25: 55.11 in
Barrel length	HK21, HK21A1, HK11A1, HK11E, HK23E, HK13E: 450 mm (17.7 in) HK21E: 560 mm (22.0 in) HK25: 35.43 in
Cartridge	HK21, HK21A1, HK11A1, HK21E, HK11E: 7.62×51mm NATO HK23E, HK13E: 5.56×45mm NATO HK25: 12.7×99mm NATO
Action	Roller-delayed blowback
Rate of fire	HK21, HK21A1: 900 rounds/min HK11A1, HK21E, HK11E: 800 rounds/min HK23E, HK13E: 750 rounds/min HK25: 450 rounds/min
Muzzle velocity	HK21, HK21A1, HK11A1, HK11E: 800 m/s (2,625 ft/s) HK21E: 840 m/s (2,755.9 ft/s) HK23E, HK13E: 910 m/s (2,985.6 ft/s) (using the SS109 cartridge) HK25: 3,000fps
Effective firing range	100–1,200 m sight adjustments
Feed system	M13, DM6, DM1 ammunition belt, 20-round detachable box or 50-round drum magazine (from the G3). An 80-round drum magazine or even a 100-round drum magazine was also available (No longer manufactured).
Sights	Rear rotary diopter drum; hooded front post

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 weapon is in use with the armed forces of several Asian, African and Latin American countries. It was also license-manufactured by Fábrica de 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 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. Thus a 4:1 transmission ratio is maintained (as long as the rollers move on the inclined surfaces of the barrel extension and locking piece) 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.