News Flash: All weapons used by the military are military weapons, by definition.
That does not make them exclusively military weapons. The Remington Model 700 bolt-action rifle has been manufactured by Remington since 1962 with the target consumer being hunters. However, in 1989 the US Army decided to adopt the rifle and designated it the M24 Sniper Rifle.
The M24 Sniper Rifle is a "military rifle." The Remington Model 700 is a civilian rifle, even though they are exactly the same bolt-action rifle.
With regard to the M16, it began as the civilian AR-15 in the mid-1950s, and sold to Colt in 1958 due to financial difficulties ArmaLite was experiencing at the time. Colt modified the AR-15, turning it into a select-fire with full-automatic capabilities and sold it to the US Army in 1964 who designated the rifle as the M16A1.
The M16A1 is a military rifle, the AR-15 is a civilian rifle. These two rifles, however, are not the same unlike the Remington example above. The M16A2 and later variants are still select-fire with a three-round burst capability. A capability the AR-15 has never had.
Keep in mind that every shotgun used by the military prior to 1972 and the Atchisson AA-12 was manufactured exclusively for civilian use before the military adopted them. Just because the military uses a firearm automatically makes it a military firearm, but it does not mean that firearm was manufactured specifically for the military. Most small arms are adopted by the military, not manufactured exclusively for them.