Staff Writer (Updated: 8/2/2016):
The StG 45 made use of the intermediate 7.92x33mm Kurz ("short") cartridge though, perhaps most importantly, it brought about the use of the "roller-delayed blowback" firing principle. This operation essentially involved two locking rollers situated to either side of the firing pin near the base of the cartridge, engaging the sides of the receiver during the firing action and delaying the movement of the bolt head - the barrel remaining fixed in place. The principle was adopted as a cost-effective alternative to a proposed gas-operated, roller-locked breech system originally intended for the StG 45. The end of the war stymied development of the StG 45 but German engineers, having relocated to Spain after the war, began perfecting the system under the government-sponsored Centro de Estudios Tecnicos de Materiales Especiales ("Technical Studio of Special Materials") otherwise known as CETME. The engineers managed to produce a more refined and reliable roller-delayed blowback system chambered for the 7.92mm cartridge and marketed it towards the West German Army for consideration as their new standard-issue service rifle. The West German Army favored the CETME idea but preferred a weapon utilizing the 7.62x51mm NATO standard cartridge instead. CETME engineers continued their work on the design and ultimately produced the CETME "Model 58" assault rifle/battle rifle firing a reduced-charge 7.62mm cartridge (to become the 7.62x51mm CETME).
After evaluation of competing systems, the West German Army selected the CETME design as their new standard-issue assault rifle. License-production of the weapon was obtained and the firm of Heckler & Koch revised the CETME system to accept the full-power 7.62x51mm NATO cartridge. The resulting design became the HK G3 series ("Gewehr 3") automatic rifle/battle rifle series which made use of sheet metal stampings in its construction dotted by furniture manufactured of plastic. The internal firing mechanism was a much more refined form of the original StG 45 roller-delayed blowback system and the overall construction of the rifle budget-friendly. The G3 shared much with the Spanish CETME but was, for all intents and purposes, its own evolution of the Stg 45 before it - a fairly straightforward, basic unassuming rifle formally categorized as a "Battle Rifle" due to its use of a full-power rifle cartridge. The HK G3 was introduced in 1959 and became the standard-issue service rifle of the West German Army. Some early HK G3s were, however, known to feature "CETME" stamped along their receivers, at least up until 1961.
At its core, the HK G3 was a fine, if unspectacular, automatic rifle development exhibiting lines that would become a standard for the HK concern for decades to come. It yielded a very utilitarian appearance but was a fully-functional end-product worthy of the rigors of the modern battlefield. The design incorporated a solid fixed stock with an angled pistol grip near the thumb-operated fire selector (safety, semi-automatic single-fire and full-automatic fire modes). The curved trigger unit was held within a hardened trigger guard. Ahead of the trigger lay the magazine well which accepted a 20-round detachable box magazine. The magazine release catch was aft of the magazine well. The receiver largely incorporated long-running horizontal lines from front to rear. The forestock consisted of a plastic handguard shrouding a good length of the barrel which protruded a distance away and was capped with a slotted muzzle flash hider. There was a rear sighting installation atop the receiver (the identifiable "drum-type" diopter sight) with the front sight added at the extreme end of the forestock. Overall, the G3 served as a very clean, "no frills" gun design.