[3] Its modular design was used for several other HK firearm models, including the HK21, MP5, HK33, PSG1, and G41.The origin of the G3 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 Maschinenkarabiner Gerät 06 (MKb Gerät 06, "machine carbine device 06") prototype assault rifle chambered for the intermediate 7.92×33mm 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.

[6] The German technicians involved in developing the StG 45(M) were taken to work in France at Centre d'Etudes et d'Armement de Mulhouse (CEAM).

Three versions were made, chambered in .30 Carbine, 7.92×33mm Kurz, and the experimental 7.65×35mm French short cartridge developed by Cartoucherie de Valence in 1948.

Engaged in the Indochina War and being the second[clarification needed] NATO contributor, France canceled the adoption of these new weapons for financial reasons.

The Modelo 2 drew the attention of the West German Bundesgrenzschutz (Border Guards), who sought to re-equip the newly formed national defense forces.

Further development of the rifle with input from H&K produced the CETME Model B which received several modifications, including the ability to fire from a closed bolt in both semi-automatic and automatic firing modes, a new perforated sheet metal handguard (the folding bipod had been the foregrip in previous models), improved ergonomics and a slightly longer barrel with a 22 mm rifle grenade launcher guide.

However, the newly formed West German Army (Bundeswehr) now showed interest and soon purchased a number of CETME rifles (7.62×51mm NATO chambering) for further testing.

The CETME, known as the Automatisches Gewehr G3 according to German nomenclature, competed successfully against the Swiss SIG SG 510 (G2) and the American AR-10 (G4) to replace the previously favored G1 rifle.

In the case of the G3, the Dutch firm Nederlandse Wapen en Munitiefabriek (NWM) held production and sales rights to the CETME design outside of Spain.

[9] Known manufacturers of the weapon included France (MAS), Greece (Hellenic Defence Systems), Iran (Defense Industries Organization), Luxembourg (Luxemburg Defense Technologie), Mexico, Myanmar, Norway (Kongsberg Våpenfabrikk), Pakistan (Pakistan Ordnance Factories), Portugal (FBP), Saudi Arabia (Military Industries Corporation (Saudi Arabia)), Sweden (Husqvarna Vapenfabrik AB and FFV Carl-Gustaf in Eskilstuna), Thailand, Turkey (MKEK) and the United Kingdom (Royal Ordnance).

Currently (2018) hundreds of thousands of G3A3A1, G3A4A1 and G3KA4A1 modernized variants rifles are maintained by Bundeswehr personnel and kept in reserve or are available in military base small arms storages.

Based on the geometric relationship arising from the angles of the roller contact surfaces of the locking piece and the barrel extension recesses, the recoil of the bolt head is delayed by a ratio of 4:1 for the 7.62×51mm NATO chambering.

The reliable functioning of roller-delayed blowback mechanisms is limited by specific ammunition and arm parameters like bullet weight, propellant charge, barrel length and amount of wear.

For obtaining a proper and safe functioning parameters bandwidth Heckler & Koch offer a variety of locking pieces with different mass and shoulder angles.

The angles are critical and determine the unlock timing and pressure curve progression as the locking pieces act in unison with the bolt head carrier.

The stamped sheet metal cocking handle tube and receiver are large exposed parts that are prone to deformation from hard impact as they were designed to be relatively thin to save weight.

To determine and correct such situations armorers are trained to employ specially designed "GO" and symmetry gauges and straightening mandrels to fairly quickly repair such problems.

From the G3A2 variant the firearm is equipped with a relatively low iron sight line that consists of a Drehvisier a rotary rear drum and hooded front post.

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 HK41, 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.

The G3 rifle is or was produced under license in the following countries: Pakistan, Brazil, Iran, France, Greece, Norway, Mexico, Myanmar, Portugal, Saudi Arabia, Sweden and Turkey.