M16, chambered in 5.56mm, is the U.S. military designation for a family of rifles derived from the ArmaLite AR-15 and further developed by Colt starting in the mid-20th century. The M16 rifle family including the M16/A1/A2/A3/A4 has been the primary infantry rifle of the United States military since the 1960s. With its variants, it has been in use by 15 NATO countries, and is the most produced firearm in its caliber. The M16 entered Army service in 1964.