Staff Writer (Updated: 5/9/2016):
The ArmaLite concern was founded in 1954 and became a developer of small arms systems, the company based out of its Illinois headquarters. One of its key people became famed gunsmith Eugene Stoner (1922-1997), who - in the eyes of many - could lay claim to his influence in the world of firearms comparable to the great John Browning himself. Stoner, who became chief engineer at ArmaLite in 1954, lent his considerable talents to the design of several promising though unfulfilled small arms productions. In 1955, Stoner developed the AR-10 rifle, a system chambering the 7.62x51mm NATO standard round, offering selective fire and being of relatively lightweight in its construction - highly suitable for use in a military environment. Seeing potential long-term revenue of the rifle in US military hands, ArmaLite entered their AR-10 design into competition to become the next US Army standard infantry rifle. However, the type was rejected in favor of the T44 prototype (a safer approach in the eyes of the US Army), eventually to become the M14 rifle -- an evolution of the World War 2-era M1 Garand.
Nevertheless, all was not lost for the future of the AR-10 for the US Army continued its interest in the AR-10 and called for some revisions to the basic design. One of the major changes was in the chambering of the smaller .223 Remington cartridge. The resulting rifle design was then designated as the "AR-15" which formally appeared in 1958. Amidst financial troubles, ArmaLite was forced to sell off its AR-10 and AR-15 designs to competing Colt and Stoner himself eventually left ArmaLite in 1961. The AR-15 was eventually adopted into the US Army inventory as the well-known "M16" while Colt also took the rifle to the civilian market as the "Colt AR-15" (a registered company trademark) and these were completed as semi-automatic rifles beginning sales in 1963
Outwardly, the AR-15 shared much of the appearance of the highly recognized M16 infantry rifle of the US Army. The receiver (specifically made up of of a lower and upper section) included all of the major internal working components including the low-set magazine feed, angled pistol grip and trigger group. The trigger was curved forward and protected by a thin ring guard. The pistol grip and magazine well supported the trigger system at the rear and front respectively. The upper receiver component managed the ejection port and cocking mechanism -both set to the right-hand side of the weapon. An optional carrying handle was well-integrated into the design and set about the upper receiver. A forend covered the barrel assembly and protected the operator's hand from the hot barrel. The barrel protruded ahead of the forend and was capped by a baffled muzzle brake. An adjustable triangular-appearing forward sight adorned the weapon aft of the barrel but forward of the forend. There was also an adjustable rear sight for precise firing. At the rear of the receiver there was fitted a shoulder stock. The magazine feed accepted various-count magazines - straight or curved in nature. The design eventually accepted a myriad of barrel lengths as well, these to suit specific operator requirements, ranging from 6.5 inches to 24 inches. Construction of AR-15s was of aluminum and synthetics making for a manageable 5.5-to-8.5lb operating weight.
As mentioned, the AR-15 was originally chambered for the .223 Remington. The firing action was of direct impingement with a rotating-lock bolt system. Direct impingement implied a gas operation system which reconstituted the erupting gasses from a fired cartridge and directed this into the bolt carrier to cycle the firing action for the next available cartridge (if any remained). Stoner proved a large proponent of the direct impingement system regarding automatic firearms. The AR-15 was an air-cooled rifle system to which the operator risked overheating the barrel through much subsequent firing without a break. Listed rate-of-fire was approximately 800 rounds per minute on full-automatic (if available on the particular AR-15 model). Muzzle velocity was rated at 3,200 feet per second. Effective range was out to 550 yards on average and depending on firing conditions. The AR-15 cartridge adaptability had since grown to include the .45 ACP, 5.7x28mm, the 6.5mm Grendel, the 6.8mm Remington SPC and .50 Beowulf with proper barrels, chamber pressurization and applicable changes to suit particular cartridge function.