The HK21 is a the belt-fed iteration of the H&K roller-delayed blowback mechanism. H&K built a wide range of firearms around that system, including submachine guns (MP5), rifles (HK33, HK91), precision rifles (PSG-1), and machine guns. Originally they made a version in 5.56mm and one in 7.62mm, but later combined both guns into a single universal platform. The standardized receiver can be fitted with either magazine-feed or belt-feed mechanisms in either 5.56mm or 7.62mm, greatly simplifying H&K’s production and logistics.
This is the 7.62mm belt fed model, which was popular with clients like Portugal who used the G3 rifle but not the MG3 machine gun. It has a reputation for unrivaled reliability under terrible maintenance and harsh environments. The downside, however, is harsher recoil than most other guns I have used in its class.
Many thanks to H&K USA for providing me access to this HK21, and to Trijicon for graciously providing use of their range and their VCOG optic!