Introduced during the Vietnam War, as a derivative of an Air Force survival rifle, the M-16, known commercially as the Colt Armalite, is, especially in evolved version, the standard battle rifle of the United States and much of the world. In the U.S. military, the most common version is a shorter-barreled M-4.
The M-16 broke several traditions in American weapons design, and indeed a number of worldwide assumptions. It is defined as an assault rifle, the most important aspects of which being that it fires an intermediate-power rifle cartridge. A "full-power" rifle cartridge would be a NATO 7.62mm caliber used by the WWII M-1 Garand, or the later M-14 rifle and M-60 medium machine gun, where submachine guns and some carbines fire a pistol round, such as the 9mm or .45 ACP. 5.56mm rounds for the M-16 are at the lower end of caliber (i.e., diameter) for a rifle cartridge, but make up for lower bullet weight by a higher muzzle velocity.
Some of the ideas that went into selecting that cartridge was the assumption that most longer-range engagements are not fought with individual rifles, but with machine guns and artillery, with long-range precision rifle fire only from specialists. The need was to fire many rounds to have the enemy take cover; there was no attempt to have "one bullet, one kill".
The smaller round let all users carry a greater number of rounds of ammunition, but also reduced the size and weight of the rifle, which made it easier for small Asians to use.
Unfortunately, the rifle was designed for a particular propelling powder charge, which was not used in the original production ammunition. This ammunition tended to foul the rifle with deposits of poweder residue. Coupled with some less than ideal metal selection of certain internal parts of the rifle, the early model M-16's acquired a bad reputation for needing scrupulous maintenance — not always practical in the field — or they might jam.
In contrast, the Soviet counterpart of the time, the AK-47 firing a reduced-power 7.62mm round, was extremely tolerant to dirt and rarely jammed. To make up for the physically larger cartridges, AK-47's had larger magazines than the M-16.
Over time, however, the 5.56mm ammunition was made to appropriate specifications, certain internal parts of the rifle were nickel-plated, and the later M-16s are reliable weapons. A machine gun, the M-249 in U.S. service but derived from the Belgian FN Minimi design, fires the same 5.56mm ammunition and became the standard squad automatic weapon. The M-249 has replaced the M-60 as a squad weapon, giving the logistical advantage of having all primary individual weapons firing the same ammunition.