 During the US trials at Aberdeen Proving Ground in 1964, the AR-18's functioning was found to vary from lot to lot of ammunition.The evaluating board concluded that while the basic design of the AR-18 was sound, it required additional minor revisions and changes to improve safety and reliability before it could be considered for adoption as a service rifle.The British Ministry of Defence (MOD) tested the AR-18 in March 1966, and found the design unsatisfactory in performance during mud and sand trials.