The basic action of the machine gun, which received the company designation HK 21, was similar to that of the G3 rifle. The HK 21 fired from a closed bolt (not that big issue since its heavy barrel was really quick-detachable) and, unlike most machine guns, its belt feeding module was located below the receiver.
HK11E automatic rifle (magazine fed, 7.62 mm)
HK13E automatic rifle (magazine fed, 5.56 mm)
HK21E general purpose machine gun (belt feed, 7.62 mm)
HK23E light machine gun (belt-fed, 5.56 mm).
The "E" stands for "Export" model.