After the M14 rifle was equipped with military, it was immediately deploy to Vietnam. However, in the jungles of Vietnam, the terrible shortcomings of the M14 rifle began to be exposed. After the US Army failed to change equipment, they learned from the experience and pinned their hopes on the development of another leading weapon - SPIW. At first, the army's top brass only regarded the M16 as a transitional weapon between the M14 and the SPIW. However, due to the failure of the SPIW, the M16 was finally made the standard rifle of the US military.
As an early type of the XM16E1 Mod 603 "early model", the "X" prefix indicates that it is an experimental number (meaning that it has not yet been officially signed contract), and it is also the first US military coded as "03" (army type, with "bolt assist knob"). into the M16. It was the first M16 series rifle to be used in the Vietnam War (1964~66). Its distinctive 3-prong flash hider has become a classic icon of the M16 series in Vietnam War.