The HK 121 fulfills the General Purpose Machine Gun (GPMG) product slot for German gunmaker Heckler & Koch. It stands as a 7.62x51mm NATO-chambered offshoot of the 5.56x45mm HK MG4 squad automatic weapon. At its core is a proven gas-operated system performing at 600 to 800 rounds-per-minute with feeding from an ammunition belt. Effective range reaches out to 600 meters with a maximum range double that - sighting through a standard diopter arrangement or iron sights. The weapon exhibits a weight of 10.8 kilograms and length of 1,165mm with a barrel length of 550mm while also showcasing very little observable recoil when fired. The HK 121 has been adopted in limited numbers by the German Army since 2005 and is intended to replace the MG3 GPMG line, itself a post-World War 2 (1939-1945) development of the MG42.
As a GPMG, the HK 121 follows accepted practice as a belt-fed, air-cooled multi-use weapon. It can be fired from the shoulder with support from a shoulder strap and foregrip, along the ground through use of a folding bipod, from a tripod assembly or from a vehicle as a suppression weapon. The design is dominated by a large rectangular receiver with an optional shoulder stock while the pistol grip and trigger unit are integral. Along the top of the receiver is a length of rail for the adoption of various optics and night vision accessories to suit both customer requirements and mission needs. The barrel is of a "quick-change" design which is required of a good air-cooled machine gun and the included carrying handle aids in its changing for a process lasting just seconds. The gas cylinder sits under the barrel in the usual way.
Vehicle versions lose the full shoulder stock and are given spade grips that interact direction with the trigger unit (the pistol grip is still retained). Portability for the infantryman is broadened by use of the shoulder strap with vertical foregrip or by way of the folding bipod. The heavy-duty tripod allows the weapon a very stable gunnery platform that reduces recoil to nil. As a belt-fed weapon, the HK 121 can use exposed ammunition belts, a 50-round drum container or a 120-round box as a feeding sourcing. Robustness is key with the HK 121 product, its operation suffering not when even fully submersed in water and fired thereafter - with no degradation of the action.
The HK 121 certainly has large shoes to fill as the MG3 is a proven winner and has been adopted in large numbers by many world armies while also license-produced by local factories is several countries.