 The reasons for this are unclear, but may have had something to do with the existing sales popularity of the AR-15/M16, as well as the need for additional field testing and evaluation of the Costa Mesa-produced rifles, which were still in the advanced prototype stage.The AR-18 was purchased for evaluation trials by various armed forces, including the United States (1964) and the United Kingdom (1966).These suffered various malfunctions during evaluation trials by various nations.