The M16 is still a general-issue rifle with the US Armed forces. It is also widely used by the US Law Enforcement agencies, either in military form, or in "civilian" semi-automatic only form. M16 rifles are used by many foreign military groups, most notably the British SAS, who preferred the M16 over the infamous L85A1 rifle, and by many others.The heart of the M16 is the direct gas system, instead of using a conventional gas piston and rod to propel bolt group back after the shot is fired. Instead, the hot powder gases are fed from the barrel and down to the stainless steel tube into the receiver. The hot gases, through the gas key, enter the hollow cavity inside the bolt carrier, and expands there, acting against the bolt carrier and the collar around the bolt body. The pressure of the gases causes the bolt carrier to move back against initially stationary bolt.

Both M16A1 and M16A2 can be equipped with underbarrel 40mm M203 grenade launcher. Standard sights of the M16A1 consist of a protected front post, mounted on the gas block. Rear sight also features an flip-up, dual aperture sights, with one smaller aperture for daylight usage, and another larger aperture for low light conditions. The range adjustments are made by the rotating knob, located just under the sight.