Machinegun developed by Heckler & Koch. Variable clip amount. Chambered for 7.62x51mm NATO.
A replacement, among others, for the M60 machinegun. It is fast-firing & somewhat-accurate.
There is also the HK23, which is chambered for 5.56x45mm NATO, and also has a shorter barrel.
"The HK21 is my machinegun! ^-^ "