View source for Heckler & Koch HK21

{{Use dmy dates|date=March 2023}}
{{Infobox weapon
| name               = HK21
| image              = File:HK 21 LMG Left and Right noBG.png
| image_size         = 300
| caption            = HK21A1 general-purpose machine gun
| origin             = [[West Germany]]
| type               = [[General-purpose machine gun]] <br /> [[Heavy machine gun]] (HK25)
<!-- Type selection -->| is_ranged          = yes
<!-- Service history -->| service            = 1961–present
| used_by            = See ''[[#Users|Users]]''
| wars               = {{ubl
|[[Vietnam War]]
|[[Football War]]<ref>{{cite magazine|title=Infantry Weapons of the Salvadoran Forces|magazine=Small Arms Review |volume=3|issue=8 |date=May 2000|first=Julio A. |last=Montes|url=http://www.smallarmsreview.com/display.article.cfm?idarticles=3006}}</ref>
|[[Lebanese Civil War]] 
|[[Portuguese Colonial War]]
|[[Shaba II]]<ref name="Congo">{{cite magazine|title=Les armes de Kolwezi|first=Jacques|last=Sicard|pages=25–30|magazine=La Gazette des armes|language=fr|url=http://fr.1001mags.com/parution/gazette-des-armes/numero-111-novembre-1982/page-28-29-texte-integral|issue=111|date=November 1982|access-date=18 October 2018|archive-date=19 October 2018|archive-url=https://web.archive.org/web/20181019075744/http://fr.1001mags.com/parution/gazette-des-armes/numero-111-novembre-1982/page-28-29-texte-integral|url-status=dead}}</ref>
| [[South African Border War]]
| [[Kurdish–Turkish conflict (1978–present)|Kurdish–Turkish conflict]] 
| [[Salvadoran Civil War]] 
| [[Operation Eagle Claw]] 
| [[Sri Lankan civil war|Sri Lankan Civil War]]
| [[Armed conflict for control of the favelas]]
| [[Boko Haram insurgency]]<ref>{{cite web |last1=Noir |first1=War |title=Militants seemingly captured a Heckler & Koch HK21A1 machine gun, ARGES SPL HGr-85 hand grenade with ARGES adapter, ammo, 7.62x39mm AK mags etc|url=https://twitter.com/war_noir/status/1628854155379986434 |website=Twitter |access-date=7 March 2023 |language=en}}</ref>}}
<!-- Production history -->| designer           = [[Heckler & Koch]]
| design_date        = 1961
| manufacturer       = [[Heckler & Koch]]<br />[[Fábrica de Braço de Prata]]<br />[[Secretariat of National Defense (Mexico)|SEDENA]]
| unit_cost          = 
| production_date    = 1961–present
| number             = 
| variants           = See ''[[#Variants|Variants]]''
<!-- General specifications -->| spec_label         = 
| weight             = HK21: {{convert|7.92|kg|lb|2|abbr=on}}<br /> HK21A1: {{convert|8.30|kg|abbr=on}}<br /> HK11A1: {{convert|7.70|kg|abbr=on}}<br /> HK21E: {{convert|9.30|kg|abbr=on}}<br /> HK11E: {{convert|8.15|kg|abbr=on}}<br /> HK23E: {{convert|8.75|kg|abbr=on}}<br /> HK13E: {{convert|8.00|kg|abbr=on}}<br /> HK25: {{convert|16.30|kg|1|abbr=on}}
| length             = HK21: {{convert|1021|mm|in|abbr=on}}<br /> HK21A1, HK11A1, HK11E, HK23E, HK13E: {{convert|1030|mm|abbr=on|1}}<br /> HK21E: {{convert|1140|mm|abbr=on|1}}<br /> HK25: {{convert|1400|mm|in|1|abbr=on}}
| part_length        = HK21, HK21A1, HK11A1, HK11E, HK23E, HK13E: {{convert|450|mm|abbr=on|1}}<br /> HK21E: {{convert|560|mm|abbr=on|1}}<br /> HK25: {{convert|900|mm|in|abbr=on|1}}
<!-- Ranged weapon specifications -->| cartridge          = HK21, HK21A1, HK11A1, HK21E, HK11E: [[7.62×51mm NATO]]<br /> HK23E, HK13E: [[5.56×45mm NATO]]<br />HK25: [[12.7×99mm NATO]]
| action             = [[Blowback (arms)#Roller-delayed|Roller-delayed blowback]], [[closed bolt]]
| rate               = HK21, HK21A1: 900 rounds/min<br /> HK11A1, HK21E, HK11E: 800 rounds/min<br /> HK23E, HK13E: 750 rounds/min<br />HK25: 450 rounds/min
| velocity           = HK21, HK21A1, HK11A1, HK11E: {{convert|800|m/s|0|abbr=on}}<br /> HK21E: {{convert|840|m/s|abbr=on|0}}<br /> HK23E, HK13E: {{convert|910|m/s|abbr=on|0}} (using the SS109 cartridge)<br /> HK25: {{convert|915|m/s|abbr=on|0}}
| range              = {{convert|100|-|1200|m|yd|abbr=on|0}} sight adjustments
| feed               = M13, DM6, DM1 ammunition belt, 20-round detachable box or 50-round [[drum]] [[Magazine (firearm)|magazine]] (from the [[H&K G3|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 [[7.62×51mm NATO|7.62&nbsp;mm]] [[general-purpose machine gun]], developed in 1961 by small arms manufacturer [[Heckler & Koch]] and based on the [[Heckler & Koch G3|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 ([[Federal Police (Germany)|''Bundespolizei'']]) it is designated "G8".

==Design details==

===Operating mechanism===
[[File:G3 Bolt.PNG|thumb|left|A schematic of the Heckler & Koch roller-delayed blowback mechanism]]

The HK21 is a [[selective fire]] [[Blowback (firearms)#Roller-delayed|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 (firearm)|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 chambering dependent (4:1 for 7.62×51mm NATO) or (3:1 for 5.56×45mm NATO) 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 head carrier travels backwards significantly 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 (firearms)|chamber]] received a series of flutes designed to help free the bloated cartridge casing from the chamber wall.
Like the G3 and HK33 assault rifle bolts the HK21 bolt features an anti-bounce mechanism that prevents the bolt from bouncing off the barrel's breech surface. The "bolt head locking lever" is a spring-loaded claw mounted on the bolt carrier that grabs the bolt head as the bolt carrier group goes into battery. The lever essentially ratchets into place with friction, providing enough resistance to being re-opened that the bolt carrier does not rebound. The spring-powered claw extractor is also contained inside the bolt while the lever ejector is located inside the trigger housing (actuated by the recoiling bolt).

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.<ref>{{Cite web|url=https://www.youtube.com/watch?v=sd8FrUe8fMM|title=How Does It Work: Roller Delayed Blowback|via=www.youtube.com}}</ref><ref>{{Cite web|url=https://www.internationalsportsman.com/how-roller-delayed-firearms-work-and-why-it-matters/|title=How Roller-Delayed Firearms Work and Why it Matters|first=Nathan|last=Thornton|date=12 October 2019|website=International Sportsman}}</ref>

===Features===
[[File:HK21.JPG|thumb|left|HK21 with ammunition belts and bipod positioned at the rear near its point of balance]]
[[File:HK-21-batey-haosef-1.jpg|thumb|left|HK21 and bipod positioned at the front for maximum stability]]

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.<ref name="Kokalis">Kokalis, Peter: ''Weapons Tests And Evaluations: The Best Of Soldier Of Fortune'', page 18. Paladin Press, 2001.</ref>
The bolt features a spring-powered [[Extractor (firearms)|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, rare for general purpose machine guns. Firing from a closed bolt contributes to accurate fire, but is disadvantageous for heat dissipation. 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 HK21 has a free-floating, heavy, quick-change barrel fitted with a slotted [[flash suppressor]]. The firearm is equipped with an [[iron sight]] line that consist of a rotary rear drum and hooded front post. The rear aperture sight is mechanically adjustable for both windage and elevation from {{convert|100|to|1200|m|yd|0}} in {{convert|100|m|yd|0|adj=on}} increments for 7.62×51mm NATO models or {{convert|100|to|1000|m|yd|0}} in {{convert|100|m|yd|0|adj=on}} increments for 5.56×45mm NATO models.

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.<ref>{{cite book|last=Cutshaw|first=Charles Q.|title=Tactical Small Arms of the 21st Century|url=https://books.google.com/books?id=qEhLZbdpkbYC&pg=PA369|year=2006|publisher=Krause Publications|isbn=978-0-87349-914-9|page=369}}</ref>

===Feeding===
The machine gun feeds from the left-hand side through a variety of disintegrating metallic link [[Belt (firearm)|ammunition belt]] types: the American [[M13 link|M13]] linked belt, the German DM6 counterpart to the M13 or the German non-disintegrating metallic link 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 (installed in place of the standard magazine well), below the barrel axis. As a result of this configuration (the bolt passes over the belt), the ammunition belt is aligned upside down compared to most other belt-fed weapons (that is, the links face downward). 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]].

==Variants==
===G8 and G8A1===
The [[German Army]], [[German Navy]] and the [[Federal Police (Germany)|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.

===GR-series===
The GR-series were "sanitized" (i.e., having no serial numbers or identifying marks) Heckler & Koch weapons used by special operations forces.{{Citation needed|date=January 2019}} 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.{{Citation needed|date=January 2019}}
*'''GR-6''' Automatic Rifle (HK13).
*'''GR-9''' Light Machinegun (HK23).

===HK21A1===
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.<ref name="Kokalis" /> The barrels were internally revised to [[polygonal rifling]]. Since then, the HK21 was offered in two main variants: the '''HK21A1''' general-purpose machine gun (with a different belt feed mechanism) and the '''HK11A1''' light machine gun. (optimized for magazine feed only).<ref name=":0">{{Cite book |title=Jane's Infantry Weapons 2002–2003 |publisher=Jane's Information Group |pages=2188}}</ref> The HK11A1 proved more successful in export sales and was adopted by the [[Hellenic Army]] and several African and Asian armies. Both weapons, as in the original HK21, can be converted to chamber the 5.56×45mm NATO round by replacing the appropriate components.

===HK21E===
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:
*'''HK11E''' light machine gun (magazine fed, chambered in 7.62×51mm NATO)<ref name=":0" />
*'''HK13E''' light machine gun (magazine fed, chambered in 5.56×45mm NATO)<ref name=":0" />
*'''HK21E''' general-purpose machine gun (belt-fed, 7.62×51mm NATO)<ref name=":1">{{Cite book |title=Jane's Infantry Weapons 2002–2003 |publisher=Jane's Information Group |pages=2194}}</ref>
*'''HK23E''' light machine gun (belt-fed, 5.56×45mm NATO).<ref name=":1" />
The "E" simply stands for "Export" model. They use STANAG rather than HK proprietary magazines and come with different optics than German military models.

The E model is a thoroughly revised variant aimed to improve the reliability, efficiency and service life of the weapon. For this the length of the receiver was increased by {{convert|25|mm|abbr=on|1}} to provide additional recoil length for the bolt group. The recoil reduction improved accuracy of fire. Reinforcing bars were added to the receiver to provide additional strength and a guaranteed 60,000 rounds minimum service life. The bolt head, extractor and locking roller holder were completely redesigned as were the bolt carrier and recoil spring assembly, belt feed unit, buffer, barrel and trigger group. The redesigned belt feed mechanism transports the ammunition belt in two steps reducing stress on the feed and ammunition belt.

Compared to the older HK21A1 and HK11A1, the modernized "Export" weapons also feature a longer barrel shroud, extended by {{convert|94|mm|abbr=on|1}} 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). The ability to use [[Heckler & Koch G3|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.
When employed in the belt-fed [[medium machine gun]] (fire support) role the HK21E provides a [[cooking off|cook-off]] rate in excess of 1,000 rounds fired in quick succession. The shock-absorbing tripod weighs {{convert|10.5|kg|1|abbr=on}}.

===HK25===
A heavy machine gun variant of the HK21 chambered in [[.50 BMG]]/12.7×99mm NATO was proposed but never went into production.<ref name="remtek">{{cite news| title =The HK 21E Machine Gun: Mission {{sic|Compa|tability|nolink=y}} Second to None|author=Gearinger, Stephen | url = http://www.remtek.com/arms/hk/mil/21/21.htm| access-date = 5 February 2015|date=December 1998 |work=Small Arms Review }}</ref>

===HK51B===
An aftermarket conversion of an HK21 general purpose machinegun into a belt-fed short carbine created by gunsmiths F. J. Vollmer & Company Inc. The barrel is 8.9 inches long, has an overall length of 21 inches, weighs about 11&nbsp;lbs. unloaded, and has a rate of fire of 950 to 1,000 rounds per minute. Fixed barrel models have a "Tropical" MP5-style handguard with ventilation slots. Quick-change barrel models have a four-position Picatinny Rail handguard (usually used with a vertical foregrip). It feeds from a 100-round HK21 belt box and uses either the American M13 or German DM6 disintegrating link belt. Vollmer only made 30 guns, 2 of which had quick-change barrels.

==Users==
<!--READ FIRST: This section is for cited entries only. Please do not add entries into this list without a citation from a reliable source. All entries without a citation will be removed. Thank you.-->

*{{flag|Argentina}}: ''HK23E'' variant used by [[Comandos Anfibios]]<ref>{{Cite web |date=24 November 2016 |title=wiw_sa_argentina – worldinventory |url=https://sites.google.com/site/worldinventory/wiw_sa_argentina |access-date=2023-02-28 |archive-url=https://web.archive.org/web/20161124203421/https://sites.google.com/site/worldinventory/wiw_sa_argentina |archive-date=24 November 2016 }}</ref>
*{{flag|Bangladesh}}: ''HK11A1'' and ''HK21A1'' variants.<ref name="jones2009">{{cite book |editor1-first=Richard D. |editor1-last=Jones |editor2-first=Leland S. |editor2-last=Ness |title=Jane's Infantry Weapons 2009/2010 |date=27 January 2009 |edition=35th |publisher=Jane's Information Group |location=Coulsdon |isbn=978-0-7106-2869-5}}</ref>
*{{flag|Brazil}}: ''HK21E'' variant issued to special forces,<ref>{{Cite web |title=ELITES OF THE EXÉRCITO BRASILEIRO – Small Arms Review |date=11 March 2011 |url=https://smallarmsreview.com/elites-of-the-exercito-brasileiro/ |access-date=2022-10-19 |language=en-US}}</ref>  ''HK21A1'' variant in use by [[Batalhão de Operações Policiais Especiais|BOPE]]<ref>{{Cite web |title=Fuzil metralhadora capaz de disparar cem tiros chama atenção durante reconstituição – Desterro1 |url=http://www.desterro1.com/2012/08/fuzil-metralhadora-capaz-de-disparar.html |access-date=2022-10-19 |website=www.desterro1.com}}</ref>
*{{flag|Bolivia}}<ref name=GANGAROSA01>Gangarosa, 2001. p. 159.</ref>
*{{flag|Brunei}}: ''HK21A1'' variant.<ref name="jones2009"/>
*{{flag|Cameroon}}: ''HK21'' variant.<ref name="jones2009"/>
*{{flag|Croatia}}: ''HK21A1'' variant<ref>https://www.morh.hr/images/stories/_morh_2018/pdf/godisnje_izvjesce_o_obrani_za_2017.pdf {{Dead link|date=February 2022}}</ref>
*{{Flag|Cyprus}}<ref>{{Cite web |title=Ιερείς σε στρατιωτική άσκηση: Έριξαν βολές και προκάλεσαν αντιδράσεις (ΒΙΝΤΕΟ) |url=https://www.alphanews.live/cyprus/iereis-se-stratiotiki-askisi-erixan-boles-kai-prokalesan-antidraseis-binteo |access-date=2023-01-14 |website=www.alphanews.live |language=el}}</ref>
*{{flag|Ecuador}}<ref name="remtek"/>
*{{flag|El Salvador}}<ref name="jones2009"/>
*{{flag|Germany}}<ref name="boßdorf">Boßdorf, Peter (2008). ''Ausrüstung der Bundeswehr''. Report Verlag. {{ISBN|9783932385261}}.</ref>
*{{flag|Greece}}: ''HK11A1'' variant.<ref name="jones2009"/> Made under license.<ref name="miller2001">Miller, David (2001). ''The Illustrated Directory of 20th Century Guns''. Salamander Books Ltd. {{ISBN|1-84065-245-4}}.</ref>
*{{flag|Jordan}}: ''HK21E'' variant.<ref name="jones2009"/>
*{{flag|Kenya}}: ''HK21A1'' variant.<ref name="jones2009"/>
*{{flag|Latvia}}: ''HK21E'' variant issued to Zemessardze National Guard units.<ref name="jones2009"/>
*{{flag|Malaysia}}: ''HK11A1'' and ''HK21E'' variants.<ref name="jones2009"/>
*{{flag|Mauritius}}: ''HK21E1'' variant.<ref name="jones2009"/>
*{{flag|Mexico}}: ''HK21A1'' <ref name="gander1995">Gander, Terry J.; Hogg, Ian V. ''Jane's Infantry Weapons 1995/1996''. Jane's Information Group; 21 edition (May 1995). {{ISBN|978-0-7106-1241-0}}.</ref> and ''HK21E'' variant.<ref name="jones2009"/> Made under license.<ref name="miller2001"/>
*{{flag|Niger}}: ''HK11'' and ''HK21'' variants.<ref name="jones2009"/>
*{{flag|Nigeria}}: ''HK21'' variant.<ref name="jones2009"/>
*{{flag|Paraguay}}: ''HK21'' variant.<ref name="paraguay">{{cite book |first=Juan Carlos|last=Wasmosy |title=Contra viento y marea: se institucionalizaron las FF. AA. de la Nación |year=2006 |publisher=Servilibro |location=Asunción |isbn=9789992535684}}</ref>
*{{flag|Peru}}<ref name="remtek"/>
*{{flag|Qatar}}: ''HK21'' variant.<ref name="jones2009"/> 
*{{flag|Senegal}}: ''HK21'' variant.<ref name="jones2009"/>
*{{flag|Sri Lanka}}: ''HK11'' and ''HK21'' variants.<ref name="jones2009"/>
*{{ZAF}}: ''HK21'' variant used by [[32 Battalion (South Africa)|32nd Battalion]] during South African Border War.
*{{flag|Sudan}}: ''HK21'' variant.<ref name="jones2009"/>
*{{flag|Sweden}}<ref name=GANGAROSA01/> Trials for [[Swedish Home Guard|Home Guard]] only.<ref>{{cite web|url=http://digitaltmuseum.se/011024246550/kulspruta-hk-21-a1|title=Kulspruta HK 21 A1|website=digitaltmuseum.se|access-date=15 May 2018}}</ref>
*{{flag|Thailand}}<ref name="remtek"/>
*{{flag|Turkey}}: ''HK23E'' variant is employed by the [[Turkish Gendarmerie]].<ref>{{cite web |url=http://www.jandarma.tsk.tr/silahlar/silahlaric.htm |title=Si̇lahlar |access-date=2014-04-02 |archive-url=https://web.archive.org/web/20141214113703/http://www.jandarma.tsk.tr/silahlar/silahlaric.htm |archive-date=2014-12-14 |url-status=dead }}</ref>
*{{flag|Uganda}}<ref name=GANGAROSA01/>
*{{flag|United Arab Emirates}}: ''HK23E'' variant.<ref name="jones2009"/>
*{{flag|United States}}: ''HK21'' was used by the [[United States Navy SEALs|Navy SEALs]] in the [[Vietnam War]]<ref>{{cite web|url=http://usssatyr-arl23.com/seal2.htm|title=UDT/SEAL Museum|website=usssatyr-arl23.com|access-date=15 May 2018}}</ref> and also used by the [[Delta Force]] in [[Operation Eagle Claw]].<ref name="remtek"/>

=== Former users ===

* {{flag|Portugal}}: ''HK21E'' variant,<ref name="jones2009" /> made under license<ref name="miller2001" /> Replaced in 2019 by [[5.56×45mm NATO|5.56×45mm]] [[FN Minimi|FN Minimi Mk3]] [[machine gun]]s.<ref>{{cite web |title=Janes &#124; Latest defence and security news |url=https://www.janes.com/article/86993/fn-herstal-secures-significant-deal-in-portugal}}</ref>
<gallery widths="200px" heights="200px">

File:US Navy 070326-N-4716P-040 Mexican navy gunners prepare to fire their weapon as amphibious assault ship USS Tarawa (LHA 1) pulls ahead to a safe position prior to a joint gunnery exercise.jpg|Mexican navy gunners prepare to fire an HK21
File:Exercise TRIDENT JUNCTURE (22600958075).jpg|Portuguese soldier with an HK21
File:Multinational Brigade at the Offensive Operation Assembly Area, Portugal, NATO Trident Juncture 15 (22092295914).jpg|A Portuguese HK21 on a tripod
File:Latvian soldiers with machine guns.jpg|Latvian soldiers with (left to right) [[Rheinmetall MG 3|MG 3]], [[FN MAG]] and HK21 general-purpose machine guns
</gallery>

==References==
=== Citations ===
{{Reflist}}

=== General bibliography ===
{{Refbegin}}
* {{Cite book |last=Gangarosa |first=Gene Jr. |year=2001 |title=Heckler & Koch: Armorers of the Free World |location=Accokeek, Maryland |publisher=Stoeger Publishing |isbn=0-88317-229-1}}
* {{Cite book |last1=Hogg |first1=Ian |last2=Weeks |first2=John S. |year=2001 |title=Military Small Arms of the 20th Century |edition=6th |location=Northbrook, Illinois |publisher=Krause Publications |isbn=0-87349-120-3}}
* {{Cite book |last=Kokalis |first=Peter |year=2001 |title=Weapons Tests and Evaluations: The Best of Soldier of Fortune |location=Boulder, Colorado |publisher=Paladin Press |isbn=978-1-58160-122-0}}
{{Refend}}

==External links==
{{Commons category|HK 21}}
* [https://web.archive.org/web/20110608033832/http://www.hkpro.com/index.php?option=com_content&view=article&id=32:the-hk21e HKPRO]

{{Delayed Blowback Firearms}}{{General Purpose Machine Guns}}
{{HK firearms}}
{{Sturmgewehr 45 derivatives}}

{{DEFAULTSORT:Heckler and Koch HK21}}
[[Category:7.62×51mm NATO machine guns]]
[[Category:General-purpose machine guns]]
[[Category:Roller-delayed blowback firearms]]
[[Category:Heckler & Koch machine guns]]
[[Category:Light machine guns]]
[[Category:Machine guns of Germany]]
[[Category:Cold War weapons of Germany]]
[[Category:Weapons and ammunition introduced in 1961]]