page_title: Heckler & Koch HK21
section_title: Operating mechanism
hierarchical_section_title: Heckler & Koch HK21 / Design details / Operating mechanism
caption_attribution_description: Cutaway view of the G3 style rifle's Roller delayed Blowback system.
context_page_description: 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 and the federal police it is designated "G8".
context_section_description: 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 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 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).
caption: A schematic of the Heckler & Koch roller-delayed blowback mechanism
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