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The development was guided by the Army during the 1950s, which culminated in a field trial in Vietnam in the early 1960s. This led to its official adoption in 1964 by the USAF as the M16. Various modified versions of the M16 design were subsequently fielded under experimental designations, culminating in the M16A1.
The M16A1 was simply the M16 with a forward assist as requested by the Army. This weapon remained the primary infantry rifle of the United States military from 1967 until the 1980s, when it was supplemented by the M16A2. The M16A2, in turn, is currently being supplemented by the M16A4, which incorporates the modular flattop receiver unit developed for the M4 Carbine. Previous versions of the weapon are still in stock and used primarily by reserve and national guard units in the United States as well as by the U.S. Air Force. The M16A3 is a fully-automatic variant of the M16A2, issued primarily within the U.S. Navy.
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