The M16 is, in my view, a battle rifle with a carbine caliber round. As a true 500 meter weapon, it sacrifices some of the range of a true battle rifle round like the 30.06 or 7.62x51 mm NATO for the lighter (and less lethal) 5.56x54mm NATO round. The down size in bullet size and powder load also considerably reduces the battlefield lethality of the M16. While one shot in the chest cavity from a 7.62 round will generally kill or incapacitate a man, it may take 2-3 or more shots from an M16. The advantages of the M-16 is that it is lighter, and the lighter round allows more rounds to be carried. The M-16 can also be fired accurately by smaller shooter such as women, who may not be able to effectively fire the M14 it replaced during the Vietnam era.
The M16 modeled after the Stoner Rifle design is a direct gas operated weapon, using the gas pressure built up behind the bullet to reset the bolt carrier assembly. The blow back gas from the barrel leads to carbon fouling in the chamber, bolt and bolt carrier assembly, requiring regular cleaning to keep the weapon functioning smoothly.
The M16 is a true rifle, with a 20 inch barrel and rifle length gas system that allows the M193 round to fully accelerate before leaving the barrel. This creates more stable ballistics at ranges exceeding 300m then those from it's shorter barreled little brother the M4 and M4A1, it also increases the lethality of the M193 and M855 ball rounds over the M4 series.
The M16 has been the service rifle of the United States for over 40 years. The initial M16 was a select fire weapon with the option of single fire (SF) or automatic fire (AF), The US military switched to the M16A2 which incorporated a forward assist, A2 style birdcage, and handguards as well as switching from AF to 3 round burst as the select fire option. The M16A3 was also adopted as an automatic version of the M16A2. M16A2s are still in service today. The M16A4 is the current version of the M16, it is similar to the M16A2 with the exception of using the KAC RAS rail system instead of the A2 hand guard, and the removal of the caring handle sight assembly which has been replaced with a standard M1913 rail. A modified caring handle sight assembly can be attached to this rail, or an optic can be used such as the TA31 ACOG used by the Marine Corps and Army. Both the A2 and A4 have the option of replacing the hand guard with the M203 40mm Grenade system, although there are options for a rail mounted 203 currently available as part of the SOCOM II accessories.
