After World War II, the
United States military started looking for a single automatic rifle to replace the
M1 Garand,
M1/M2 Carbines,
M1918 Browning Automatic Rifle,
M3 "Grease Gun" and
Thompson submachine gun. However, early experiments with select-fire versions of the M1 Garand proved disappointing. During the
Korean War, the select-fire
M2 Carbine largely replaced the
submachine gun in US service and became the most widely used Carbine variant. However, combat experience suggested that the
.30 Carbine round was under-powered. American weapons designers concluded that an
intermediate round was necessary, and recommended a small-caliber, high-velocity cartridge. However, senior American commanders having faced fanatical enemies and experienced major logistical problems during WWII and the Korean War, insisted that a single powerful .30 caliber cartridge be developed, that could not only be used by the new automatic rifle, but by the new
general-purpose machine gun (GPMG) in concurrent development. This culminated in the development of the
7.62×51mm NATO cartridge. The ArmaLite AR-10 featured an innovative combination of a straight-line barrel/stock design, a new patent-filed gas-operated bolt, forged aluminum alloy receivers and with
phenolic composite stocks resulting in a small arm significantly easier to control in automatic fire than other infantry rifles of the day. It had rugged elevated sights, an oversized aluminum
flash suppressor and
recoil compensator, and an adjustable gas system. The final prototype, featured an upper and lower receiver with the now-familiar hinge and takedown pins, and the charging handle was on top of the receiver placed inside of the carry handle. In the end, the United States Army chose the T44, which entered service as the
M14 rifle, The U.S. also adopted the
M60 general purpose machine gun (GPMG). While the M2 Carbine offered a high rate of fire, it was under-powered and ultimately outclassed by the AK-47. A replacement was needed: a medium between the traditional preference for high-powered rifles, such as the M14, and the lightweight firepower of the M2 Carbine.
Scaling down the ArmaLite AR-10 As a result, the Army was forced to reconsider a 1957 request by General
Willard G. Wyman, commander of the U.S. Continental Army Command (CONARC), to develop a .223 caliber (5.56 mm) select-fire rifle weighing 6 lb (2.7 kg) when loaded with a 20-round magazine. The 5.56mm round had to penetrate a standard U.S.
M1 helmet at and retain a velocity in excess of the speed of sound, while matching or exceeding the wounding ability of the .30 Carbine cartridge. This request ultimately resulted in the development of a scaled-down version of the
ArmaLite AR-10, called the ArmaLite AR-15 rifle. and soldiers armed with AR-15s could also carry three times more ammunition than those armed with M14s (649 rounds vs. 220 rounds). Colt would go on to market the Colt ArmaLite AR-15 rifle to military services around the world. In July 1960,
General Curtis LeMay, then Vice Chief of Staff of the United States Air Force, was impressed by a demonstration of the AR-15 and ordered 8500 rifles. In the meantime, the Army would continue testing the AR-15, finding that the intermediate cartridge .223 (5.56 mm) rifle is much easier to shoot than the standard 7.62×51mm NATO M14 rifle. In the summer of 1961, General LeMay was promoted to Chief of Staff of the United States Air Force and requested an additional 80,000 AR-15s. However, General
Maxwell D. Taylor, now
Chairman of the Joint Chiefs of Staff, (who repeatedly clashed with LeMay) advised President
John F. Kennedy that having two different calibers within the military system at the same time would be problematic and the request was rejected. In October 1961, William Godel, a senior man at the
Advanced Research Projects Agency, sent 10 AR-15s to South Vietnam. The reception was enthusiastic, and in 1962, another 1,000 AR-15s were sent. After minor modifications, In March 1964, the M16 rifle went into production and the Army accepted delivery of the first batch of 2129 rifles later that year, and an additional 57,240 rifles the following year. The 601 and 602 are virtually identical to the later M16 rifle without the forward-assist. Like the later M16 rifle their charging handle was re-located from under the carrying handle like AR-10 to the rear of the receiver. The only major difference between the 601 and 602 is the switch from the original four grooves, right-hand 1:14-inch (1:355.6 mm or 64 calibers)
rifling twist rate to the more common four grooves, right-hand 1:12-inch (1:304.8 mm or 54.8 calibers) twist. This was done as the original 1:14-inch twist rate to just stabilize the 55 grain bullet used in the 5.56 Ball M193 cartridge under unfavorable conditions could induce too much yaw and due to the resulting excessive in flight bullet destabilization become inaccurate. The one turn in 12 inches increased accuracy and was optimized to adequately stabilize the M193 ball and longer M196 tracer bullets. File:Colt ArmaLite AR-15 Model 01. Left Side SPAR1372 DEC. 22. 2004.jpg|Colt ArmaLite AR-15 Model 01 with 20-round magazine, made from 1959 to 1964 File:Colt Armalite AR-15 Model 02 Left Side SPAR1373 DEC. 22. 2004.jpg|Colt ArmaLite AR-15 Model 02 without magazine and new 1 in rifling twist rate, made in 1964 ==Design details==