The name 'AR-15' today is used almost exclusively to refer to the semi-automatic (commercially available) civilian version(s) of the M16 and M4 assault rifles, going from the name of a single rifle to the generic name of a type of rifles that trace their ancestry to it. The rifle would later be adopted by the United States military under the name 'Rifle, Caliber 5.56 mm, M16'. In 1959, ArmaLite sold the rights to the AR-10 and the AR-15 to Colt due to financial difficulties, and Colt continued development on the rifle. The original prototype AR-15 was developed by ArmaLite in 1956, and is a scaled down version of the ArmaLite AR-10 rifle, chambered in 5.56x45mm rather than 7.62x51mm. The M16 rifle series is the United States military designation of rifle variants of the ArmaLite AR-15 assault rifle adopted by the US military. ArmaLite AR-15, first pre-production model, serial number XAR1501 - 5.56x45mm
