In 1955, the US Army was holding trials for a battle rifle to replace their M1 Garands with a new weapon capable of fully automatic fire and being magazine fed. The new weapon was intended to either fully or partially replace the M1 Carbine, Browning Automatic Rifle (BAR), and M3 Submachine gun as well. The US Army was against any reduction in power in their new weapon despite arguments that the full power round was not suitable for automatic fire. Through the Army's dominant position, a slightly shorter cartridge with similar power to the current .30-06 rifle round became the standard 7.62x51mm NATO round. Springfield Armory and Fabrique Nationale both entered rifle designs based on fairly conventional engineering and the Spingfield M14 was selected. This rifle replaced the Garand, but was essentially a product-improved M1 Garand.
Eugene Stoner's ArmaLite 7.62x51mm AR-10 came too late in the evaluation for serious consideration, but his advanced design was a dramatic improvement on conventional thinking. Using a number of advanced features and materials, the AR-10 was almost two pounds lighter than the M14. One of the most novel concepts of the AR-10 was a composition aluminum/steel barrel. Unfortunately, during endurance testing of the AR-10 at Springfield Armory, an AR-10 suffered a burst barrel. ArmaLite realized a change was in order, and switched to a steel barrel, but it was too late. The M14 won the selection process over both the AR-10 and the Belgian FAL rifle, but the M14's victory would be short lived. The Army quickly learned that a full power 7.62mm rifle fired on full auto was an uncontrollable beast. With the United States involvement in the close quarters jungle fighting of Vietnam, a lighter rifle with increased firepower and greater control in full-auto was seen as more desirable.
Only two years later, the US Army was once again looking for a new rifle - this time based on a much smaller .22 caliber round and lighter in weight. This time, ArmaLite was invited to compete right from the start, and responded with a smaller version of the AR-10, the AR-15. Winchester Arms submitted a remake of a 'Carbine' Williams prototype carbine, and Springfield Armory was forbidden to participate, as senior Ordnance Corps officials had not yet accepted the small-caliber, high-velocity concept.
In the end the Army selected the AR-15, and after initial fielding in 1965, the M16A1 went on to become the new standard rifle in February 1967. This was not without further controversy, as early reports of problems with gas system fouling producing malfunctions, along with extraction issues which was likely caused by an overly dirty and under-lubricated weapon. Although these problems were later traced to changes in the ammunition powder and a lack of cleaning supplies and proper maintenance training in the field, it was often used as evidence against the weapon itself.