The M16 Assault Rifle is the standard-issue shoulder weapon in the U.S. military. Designed to fire small, high-velocity rounds (5.56 mm caliber vs. 7.62 mm), the weapon is relatively small and light, thereby significantly decreasing the overall load warfighters needed to carry.
The M16 is based on a design (the Colt AR-15) that had already been rejected by the Chief of Staff of the Army in favor of the heavier 7.62 mm M14. 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 this led to a the first large-scale procurement in 1966 of a modified AR-15—the M16—for deployment in the Vietnam conflict.