Rifle, American type, 5.56mm. Been in service since the Vietnam days, in various configurations.
Based on the AR15 design by the designer Stoner, manufactured by Colt and later FN. Never manufactured by Mattel, contrary to popular belief.
Rotating bolt, considered a 'dirty' weapon because of the direct application of the gas to the bolt without the use of a piston. Originally designed to be a .223 Remington calibre, it was re-designed to take the NATO standard loadings, which are a slightly higher pressure although of practically identical dimensions.
Upon initial service entry, it attained a reputation for unreliability. This wasn't the fault of the design as much as the fault of the Army trying to cut costs by using a lower grade and dirtier propellant, and also myths such as 'The M-16 never needs cleaning' Eventually these problems were rectified.
The M-16A1 had smooth forward handguards, and was select-fire between semi-auto and full auto using a three-position safety/selector switch, manipulated with the firer's thumb. The M16A2 changed the full-auto selector to three-round burst, and changed the handguard to a ribbed form.
The latest full-sized M-16 is the A4 variant, with a removable carrying handle/sight with a picatinny rail for easy addition of optics or other fun doodads that anyone wishes to put on.
Air force rifles do not come equipped with a forward assist.
The carbine version is the M-4, with a telescoping stock and shorter barrel.
The two major 'pluses' of the rifle are the magazine change and adaptability. The magazine is dropped free with the trigger finger, the replacement mag is inserted without any more manipulation and the bolt released with a slap.
The other 'plus' is the customisability. The easily-replaced handguards can have up to four mounting rails in addition to any on the top of the body. Many American soldiers have been known to place personally-owned items on their rifles, from Surefire lights and holographic sights through to the issued night vision devices or infra-red pointers.
The 5.56mm round is questioned by some in American circles because of a stated lack of immediate lethal effect. This may be a reflection on the lower muzzle velocity imparted by the shorter carbine which is in increasingly greater circles.