 The U.S. military's later adoption of the AR-15 gave legitimacy to its 5.56mm cartridge, and ArmaLite sought to develop a competing design chambered in 5.56mm that did not infringe on the Colt licence agreement.With Stoner gone, it was decided to scale down the AR-16, and ArmaLite's new chief designer, Arthur Miller, embarked on the project.The resulting 5.56mm design appeared in 1963 and was named the AR-18.