The Colt Model XM16E1 was the first attempt at providing an improved M-16 for the US Army. The Colt Model XM16E1 production began in early 1964 and transitioned into the Colt Model M16A1 in early 1967 with over 500000 being produced. It was the first model to incorporate the forward assist feature at the insistence of General Wheeler, Army Chief of Staff. This model was also the first to use the Partial Fence lower that allowed the front push pin to be retained within the receiver