[11] In 1959, ArmaLite sold its rights to the AR-15 to Colt due to financial difficulties and limitations in terms of manpower and production capacity.

[18] American weapons designers concluded that an intermediate round was necessary, and recommended a small-caliber, high-velocity cartridge.

[19] However, senior American commanders having faced fanatical enemies and experienced major logistical problems during WWII and the Korean War,[20][21][22][23][24] 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.

ArmaLite entered the competition late, hurriedly submitting several AR-10 prototype rifles in the fall of 1956 to the United States Army's Springfield Armory for testing.

[27] 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.

[28] It had rugged elevated sights, an oversized aluminum[29] flash suppressor and recoil compensator, and an adjustable gas system.

[31][32] In the end, the United States Army chose the T44, which entered service as the M14 rifle,[25] which was an improved M1 Garand with a 20-round magazine and automatic fire capability.

Battlefield reports indicated that the M14 was uncontrollable in full-auto and that soldiers could not carry enough ammo to maintain fire superiority over the AK-47.

[37] 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.

[13] The 5.56mm round had to penetrate a standard U.S. M1 helmet at 500 yards (460 meters) and retain a velocity in excess of the speed of sound, while matching or exceeding the wounding ability of the .30 Carbine cartridge.

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.

[43][44] 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.

[45] In October 1961, William Godel, a senior man at the Advanced Research Projects Agency, sent 10 AR-15s to South Vietnam.

[2][46] United States Army Special Forces personnel filed battlefield reports lavishly praising the AR-15 and the stopping-power of the 5.56 mm cartridge and pressed for its adoption.

[33] By intentionally choosing a slow twist rate the 55 grain bullet used in the 5.56 Ball M193 cartridge was only just stable in flight.

[33] Even President Kennedy expressed concern, so McNamara ordered Secretary of the Army Cyrus Vance to test the M14, the AR-15 and the AK-47.

McNamara ordered its adoption, despite receiving reports of several deficiencies, most notably the lack of a chrome-plated chamber.

However, the Army insisted on the inclusion of a forward assist to help push the bolt into battery in the event that a cartridge failed to seat into the chamber.

The Air Force, Colt and Eugene Stoner believed that the addition of a forward assist was an unjustified expense.

[53][54] The one turn in 12 inches increased accuracy and was optimized to adequately stabilize the M193 ball and longer M196 tracer bullets.

Because recoil does not significantly shift the point of aim, faster follow-up shots are possible and user fatigue is reduced.