Did some internet sleuthing tonight and I found this. It's basically covering what led up to the adoption of the AR15. Good read, somewhat long however.
1948. U.S. Army's Operations Research Office (ORO) conducts a research about small arms effectiveness. This research was completed by the early 1950 with the conclusion that the most desirable infantry small arms should be of 22 caliber, select-fire and with high velocity bullets, effective up to 300 meters or so.
1953 - 1957. US DOD conducts the next research, "Project SALVO", that also lead to the desirability of .22 caliber high-velocity infantry rifle
1957. The US Army requests the Armalite Division of the Fairchild Aircraft Corp to develop a rifle of .22 caliber, lightweight, select-fire, and capable to penetrate the standard steel helmet at 500 meters. The Eugene Stoner, then a designer at the Armalite, began to develop this rifle, based on his earlier design, 7.62mm AR-10 battle rifle. At the same time, experts at the Sierra Bullets and the Remington, in conjunction with Armalite, began do develop a new .22 caliber cartridge, based on the .222 Remington and .222 Remington Magnum hunting cartridges. This development, initially called the .222 Remington Special, was finally released as .223 Remington (metric designation 5.56x45mm).
1958. Armalite delivers first new rifles, called the AR-15, to the Army for testing. Initial tests display some reliability and accuracy problems with the rifle.
1959. Late that year Fairchild Co, being disappointed with the development of the AR-15, sold all rights for this design to the Colt's Patent Firearms Manufacturing Company.
1960. Eugene Stoner leaves the Armalite and joins the Colt. The same year Colt demonstrated the AR-15 to the US Air Force Vice Chief of Staff, Gen. LeMay. Gen. LeMay wanted to procure some 8 000 AR-15 rifles for US AF Strategic Air Command security forces to replace ageing M1 and M2 carbines.
1962. US DoD Advanced Research Projects Agency (ARPA) purchases 1000 AR-15 rifles from Colt and sends those rifles to the South Vietnam, for field trials. Same year brings glowing reports about the effectiveness of the new "black rifle", used by South Vietnamese forces.
1963. Colt receives contracts for 85 000 rifles for US Army (designated as XM16E1) and for further 19 000 rifles for US Air Forces (M16). The US AF M16 was no more than an AR-15 rifle with appropriate markings. The XM16E1 differed from AR-15/M16 by having an additional device, the so called "forward assist", which was used to manually push the bolt group in place in the case of jams.
1964. US Air Forces officially adopted new rifle as M16. Same year US Army adopted the XM16E1 as a limited standard rifle, to fill the niche between discontinued 7.62mm M14 rifle and the forthcoming SPIW system (which newer got past the prototype and trial stages).
1966. Colt was awarded with the contract for some 840 000 rifles for US Armed forces, worth almost $92 millions.
1967. US Army adopted the XM16E1 rifle as a standard "US Rifle, 5.56mm, M16A1", on 28 February 1967.
1965 - 1967. Field reports from Vietnam began to look much more pessimistic. M16 rifles, issued to US troops in the Vietnam, severely jammed in combat, resulting in numerous casualties. There were some causes for malfunction. First of all, during the introduction of the new rifle and its ammunition into the service, US Army replaced originally specified Dupont IMR powder with standard ball powder, used in 7.62x51mm NATO ammunition. The ball powder produced much more fouling, that quickly jammed the actions of the M16 unless the gun was cleared well and often. This pitifully combined with the fact that the initial M16 rifles were promoted by the Colt as "low maintenance", so, for the sake of economy, no cleaning supplies were procured for new M16 rifles, and no weapon care training was conducted fro the troops. As a result, soldiers did not knew how to clean their rifles, and had no provisions for cleaning, and thing soon turned bad. To add the trouble, the ball powders also had a different pressure curve, so they produced higher pressures at the gas port, giving the rise to the rate of fire, and, thus, decreasing accuracy and increasing parts wear.
1967 - 1970. The deficiencies discovered in previous years began do dissolve. 5.56mm ammunition was now loaded using different powders that produce much less residue in the gun action. The barrel, chamber and bolt of the rifles were chrome-lined to improve corrosion resistance. Cleaning kits were procured and issued to troops, and a special training programs were developed and conducted ever since. Earliest cleaning kits could be carried separate from rifle only, but since circa 1970 all M16A1 rifles were manufactured with the containment cavity in the buttstock, that held the cleaning kit. At the same time (circa 1970) the new 30 rounds magazines were introduced into service instead of the original 20 rounds ones, to equal Soviet and Chinese AK-47 assault rifles, which had 30-rounds magazines from the very beginning.
1977 - 1979. NATO trials lead to the adoption of the improved 5.56x45mm cartridge, developed in Belgium by FN. This cartridge, initially developed in conjunction with the FN Minimi light machine gun, featured a slightly heavier bullet with accordingly slightly lower muzzle velocity. The resulting long-range performance, however, improved due to the better ballistic coefficient of the new bullet. The SS109 required a faster rifling twist to stabilize its bullet, than the original 5.56x45mm US M193 ammunition. The M193 was used with barrels rifled with 1:12 twist (1 turn in 12 inches), and SS109 was preferred to be fired with 1:7 twist (1 turn in 7 inches). Some arms manufacturers preferred to make their guns with intermediate 1:9 rifling, which would be equally good (or bad) for both old and new loadings.
1981. Colt developed a variation of the M16A1, adapted for the SS109/5.56mm NATO cartridge, and submitted it to the military trials as the M16A1E1. This rifle differed from the M16A1 by having the heavier barrel with faster 1:7 rifling, a different type rear sights (adjustable for both range and windage), round handguards instead of triangular ones, and by replacing the full-auto fire mode with the burst (limited to 3 rounds per trigger pull), to preserve the ammunition.
1982. M16A1E1 is type-classified by US DoD as the "US Rifle, 5.56mm, M16A2".
1983. US Marine Corps adopted the M16A2 rifle.
1985. US Army officially adopted the M16A2 as the general issue infantry rifle.
1988. The FN Manufacturing Co, an US subsidiary of the FN Herstal (Belgium), becomes the key contractor to US DoD for production of the M16A2 rifles. Colt continues the development and manufacture of the AR-15 / M16 type rifles only for civilian and law enforcement markets from that point.
1994. Adoption of the latest variations of the M16 breed. Those include: M16A3and M16A4 rifles, with "flat top" receivers, that had a Picatinny accessory rails in the place of the integral carrying handle. The rail can be used to mount detachable carrying handle with iron rear sights, or various sighting devices (Night/IR, optics etc). The M16A4 otherwise is similar to M16A2, while M16A3 has a full-auto capability instead of the 3-rounds burst. Two other newest AR-15 offsprings are the M4 and M4A1 carbines, which are described in the separate article on this site.
That guy has not updated since April, the slacker, hehe.