During the Vietnam War, the United States military replaced the M14 with the M16 for a variety of reasons, one of the most important being the advantage of increased rate of fire, light weight, and lower recoil of the 5.56×45mm NATO round over the larger 7.62×51mm NATO round. Despite the fact that this shortened the effective range of the average infantryman during a firefight, the 5.56×45mm round (SS109) eventually became the standard of ammunition type for all NATO member assault rifles. In accordance with this, the Japanese Defense Agency began development on their next generation assault rifle to replace the 7.62×51mm Type 64 battle rifle after its 25-year span of service.[6][9]