The US Army long had a deeply entrenched and historical view which argued that carefully aimed, long-range rifle fire is superior to the high-volume but largely unaimed bursts of automatic weapons. Those who argued for aimed fire believed that good marksmanship and judicious control win battles and conserve ammunition.
Military historian S.L.A. Marshall found that only 4 out of 10 World War II veterans fired at the enemy. Retired Lieutenant Colonel Dave Grossman, in his book, On Killing: The Psychological Costs of Learning to Kill in War and Society, concluded that failing to fire at the enemy is a universal problem. He observes that humans have an deep aversion to harming others, and only tough, realistic training that conditions soldiers to kill relexively in combat can overcome this aversion, but even this is never completely effective. Even with such training, very few troops are mentally prepared to direct aimed fire at the enemy, preferring instead to spray bullets in the general direction of the enemy.
This is the basis of the argument that weapons with high volumes of fire would meet the wartime needs of the US military much better. At a time when the infantry of many armies is armed with sophisticated and fully automatic versions of assault rifles, US foot soldiers are equipped with the M-16, a one-pull, three-shot rifle.
The M-14 was the Army' s original choice to replace World War II-era M-1 and Browning Automatic rifles. The M14 was an unhappy compromise weapon, that satisfied virtually no one, least of all the men for whom it was intended. General dissatisfaction with the M14 and numerous studies led the Army to the development of a light weight weapon capable of firing a burst of small caliber bullets with a controlled dispersion pattern. Unfortunately, the M-14's follow-on initially fared little better.
The replacement for the M14 was originally designed by Eugene Stoner, of the ArmaLite Company, as the AR-15 around 1956. The AR-10 was conceived by Eugene Stoner as a 7.62mm Basic infantry rifle in 1955. At that time the Army was considering replacements for the M1 Garand. The AR-10 was stunningly different than any previous design. It was produced with aircraft grade aluminum receivers, and therefore weighed less than seven pounds. The stock and other furniture were plastic, while the T-44 and T-48 were of wood. The configuration of the rifle itself, with its integral carrying handle and charging handle distinctively mounted within it, sparked intense curiosity. In the end, the AR-10 wasn't able to catch up, and eventually lost out to the M-14 rifle in 1959.
Based on the AR-10, Army officials asked ArmaLite to develop a smaller version of the AR-10 in 1956. The ensuing rifle was called the AR-15. Army analysis of battlefield statistics from WWI, WWII and Korea, had shown that most kills from small arms occured at ranges of less than 300 yards. This suggested that the military should seriously consider lighter weight, higher capacity weapons. Seeking a novel cartridge suitable for a smaller caliber assault rifle, Eugene Stoner approached Winchester Corporation. The result was the small but powerful .223 Rifle Cartrdige -- high-velocity, light weight, low recoil, and capable of penetrating a helmet per US Army specifications.
Production of the AR-15 rifle was licensed to to Colt Manufacturing Company in 1959. Early Colt AR-15s, their magazines, and their operators manuals were marked with ArmaLite's name. Colt's retained the AR-15 designation on commercial rifles. To this day Colt's has a model designation with the letters AR, which stands for "ArmaLite".
The AR-15 was selectable for full and automatic fire. The AR-15 was to have had the same effective range as the M14 rifle, but it was most effective at a range of 215 yards (200m) or less. The M16 used a 5.56mm (.223 cal.) cartridge in 20- or 30-round magazines. To compensate for the reduced size of the 5.56 mm bullet, the AR-15 designers increased the velocity of the bullet so that it would have an adequate range and the flat trajectory needed for accurate aiming. The M16 bullet had a muzzle velocity (velocity on leaving the gun) of 980 meters per second as compared to 870 meters per second for the M14 rifle and 720 metres per second for the Soviet AK-47 7.62 mm rifle, while at a range of 100 meters the velocities of the three bullets were 830, 800, and 630 meters per second respectively.
The U.S. Air Force completed tests of the AR-15 in January 1961. The Air Force procured 8,500 rifles in 1961 and standardized the AR-15 in 1963. The weapon was first deployed to the Air Force's Air Police. The original AR-15 was designated the M16 in 1962.
The new rifle had the advantage from a military point of view of weighing one- fourth less than the M14, and the ammunition also was lighter, reducing the recoil against the soldier's shoulder and enabling a soldier to carry more rounds. As interest in the problems of counter-insurgency grew under the Kennedy administration in the early 1960s, the US military quietly bought several thousand AR-15s and sent them to Vietnam for testing in combat conditions.
In the Vietnam era, DARPA (then ARPA) gained acceptance for the AR-15 by sponsoring its demonstration in combat. Colt brought the weapon to DARPA in 1962. Through Project AGILE, DARPA purchased 1,000 AR-15s and issued them to combat troops in Southeast Asia for field trials, to prove that the high-velocity 5.56 mm round had satisfactory performance. The subsequent DARPA report, documenting the lethality of the AR-15, was instrumental in motivating the Secretary of Defense to reconsider the Army's decision and eventually adopt a modified AR-15 as the US military individual weapon of choice. Although opposed by the Ordnance Corps, the Armalite AR-15 was adopted by the Secretary of Defense as the 5.56mm M16 rifle.
Soon reports began appearing of the lethality of the new rifle. Unofficial reports said the AR-15's light bullet, travelling at 3,300 feet per second, did cartwheels as it penetrates living flesh, causing a highly lethal wound that looks like anything but a caliber .22 hole, the US magazine Army reported in August 1963. Two US Army doctors who evaluated AR-15 wounds at an Army hospital in South Vietnam in 1966 reported that while wounds inflicted at close range had small entrance and exit holes, those at larger ranges exhibited small entrance holes whereas the exit wound is a gaping, devastated area of soft tissue and even bone, often with loss of large amounts of tissue, with disintegration of the bullet and minute splattering of lead.
By 1963 US Army was purchasing the M16 for use in South East Asia and by various elite forces. The Army also ordered 85,000 rifles in 1963. An additional 35,000 were ordered in 1964, 100,000 in 1965, and 100,000 in 1966. These rifles were initially issued primarily to combat troops in the Dominican Republic and to Special Forces, Airborne, helicopter crews, Air Commando and other special category troops in Vietnam.
The M-16 was type classified standard A in 1965 and became the military's basic service rifle. By 1966 it was in widespread use. The M16 was called the "black rifle" and "Mattel toy" thanks to its appearance. Troops liked the light weight, but complained about insufficient range and lethality. While the M16 had been marketed as virtually "maintenance free, poor maintenance instructions (or even no instructions) and jungle climate together with the fouling-prone direct gas system caused trouble. Its high rates of fire in the jungle environment had a larger impact on increasing American morale than on actually inflicting enemy casualties. The move to high-velocity 5.56 mm was also subsequently adopted by the Israelis, the Soviets, and NATO allies. DARPA's most significant contribution to this program was its willingness to "think outside of the box" and try something new.
The AR-15 was redesignated by the US Army as the M16 rifle, and in 1967 the Army announced that it would be adopted as the standard infantry weapon for US Forces outside NATO. By 1978 the rifle had been exported to 21 countries and was being produced under licence in another three, with various other 5.56 mm rifles in production elsewhere.