The origin of this rifle can be traced back to the final years of World War II when Mauser engineers at the Light Weapon Development Group (Abteilung 37) at Oberndorf am Neckar designed the MKb Gerät 06 (Maschinenkarabiner Gerät 06 or "machine carbine device 06") prototype assault rifle chambered for the intermediate 7.92x33mm Kurz cartridge, first with the Gerät 06 model using a roller-locked short recoil mechanism originally adapted from the MG 42 machine gun but with a fixed barrel and conventional gas-actuated piston rod.
G3A3 German sniper in Afghanistan
Apart from the G3A3 and G3A4 HK also built: the G3A3ZF (essentially a G3A3 with a Hensoldt 4x24 optical sight), the accurized G3SG/1 rifle (hand-selected G3A3’s, equipped with an improved trigger, Zeiss telescopic sight with a variable 1.5-6x magnification and a cheek riser) and the G3K carbine which uses an HK33 handguard and a short barrel (reduced in length to the base of the front sight post), that is too short for use with a bayonet or rifle grenades.
The G3 served as a basis for many other weapons, among them: the PSG1 and MSG90 precision rifles, the HK11 and HK21 family of light machine guns, a semi-automatic version known as the HK91, a "sporterized" model called the SR9 (designed for the civilian market in countries where the HK91 would not qualify, primarily the US after the 1989 importation restrictions) and the MC51 carbine, produced by the UK firm FR Ordnance International Ltd. for special forces. The MC51 weighs 3.1 kg (6.8 lb), has a folded overall length of 625 mm (24.6 in), a barrel length of only 230 mm (9.1 in), which produces a muzzle velocity of approx. 690 m/s (2,263.8 ft/s) and a muzzle energy of 2215 J. The MC51 was allegedly manufactured for the British SAS and SBS, who required a compact but powerful weapon, for situations in which the stopping power and armor piercing capabilities of 9x19mm Parabellum round were inadequate. Only 50 weapons were produced, and all were reportedly shipped to the UK special forces. Most of them were soon replaced by the Heckler & Koch HK53 carbine. Another UK-based company called Imperial Defence Services Ltd. absorbed FR Ordnance and continues to market the MC51 standard variant.