Traditionally, the US Military designates the year that the weapon was accepted, as the Model number. Such as the Model 1911 Colt .45 (automatic) pistol, or the M-1903 Springfield (bolt action) rifle. In the case of the M-16 rifle, Armalite manufactured 14 firearms prior to their AR-15; thus, when the military adapted it, they added military modifications to it, making it different from the AR-15, therefore it was a 16th AR type firearm. Model (M stands for Military Model) 16.
The M16 sieries of rifles are individual combat weapons.
the M16 assault rifle is composed of steel, aircraft grade aluminum, composite plastics, and polymer
Depends where you are and what gun regulations are in place.
M16
The standard assault rifle for the US Armed Forces is the M16, the Marines first created the M16A4 the newest version of the M16, before 2011 the US Army were using the M16A3 but now they are adapting the M16A4. Although the M4A1 carbine is being used more by the US military now then the M16. In about 10 years the M4 is to be replaced.