The M16 is an American-designed 5.56x45mm assault rifle, developed in 1957 by Eugene Stoner at ArmaLite (he sold the design rights to Colt just before the US military adopted it) and based on his previous AR-10 design.
It has been the standard weapon of the US military since 1964, when it began to replace the M14 battle rifle. Prototypically distinguishable by its triangular front sight and integrated carrying handle/rear sight, although newer variants (such as the shortened M4 carbine version) sport removable units.
Had some initial bad notoriety due to the original design's tendency to jam with dirt or in some weather conditions, but is also regarded as a very effective assault rifle because of its low weight and high accuracy.
For artistic purposes the M16 is indistinguishable from the civilian AR-15 so both tags should be used.