The HK 21 is a machine gun of German origin. It was developed by Heckler & Koch in the early 1960's to complement the successful G3 battle rifle. The HK 21 is derived from the G3, which has resulted in a weapon with unique features. The HK 21 and its variants have a limited weight, are very accurate and can be easily reconfigured in caliber and feed mechanism, the downside is their high cost and limited ability for continuous fire. These properties made the HK 21 a niche product rather than a true general purpose machine gun.

The HK 21 is derived from the G3 battle rifle and uses the same roller locked delayed blowback method. In order to turn the G3 in a machine gun a quick change barrel and belt feed mechanism are added, as well as numerous other changes. The HK 21 variants come in three production models. The originals models from 1961 are rather basic and feature a G3 stock and non-adjustable bipod. The A1 models have and improved belt feed mechanism and redesigned stock. The Export models introduced in 1981 have a longer forearm and barrel, futher improved feed mechanism and new sights and an adjustable bipod. The HK 1x models are box fed and the HK 2x models are belt fed. The x1 models fire the 7.62x51mm round and the x3 models the 5.56x45mm round. All models can be reconfigured in caliber and feed mechanism.

Most HK 21 are chambered for the 7.62x51mm NATO round. Another option is the 5.56x45mm round, with the M193 used in early models and the NATO round in late models. Very small quantities of the early models were made to fire the 7.62x39mm Soviet round. The HK 21 is select fire and has a cyclic rate of fire of 750 to 900 rpm, depending on model. The belt fed models are most common. The magazine fed models use G3, HK 33 or STANAG magazines. The effective range is about 400 meters against point targets and 800 meters against area targets. When mounted on a tripod targets at even further ranges may be engaged. Due to its accuracy the HK 21 may also be used as a designated marksman rifle, especially when fitted with optics.