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.