The M16 is a lightweight, semi-automatic rifle that was first introduced into the United States military in 1963. The development of the M16 began in the late 1950s, when the U.S. Army began to explore the possibility of replacing its existing M14 rifle with a lighter, more modern weapon.

The development of the M16 was largely influenced by the experiences of American soldiers during the Korean War and the early years of the Vietnam War. During these conflicts, U.S. troops frequently encountered heavily-armed North Korean and Chinese soldiers armed with AK-47 rifles, which were far more effective than the American M1 Garand and M14 rifles.

In 1958, the U.S. Army contracted with the Armalite Corporation, a small arms manufacturer, to develop a new lightweight rifle that could fire the newly-developed 5.56mm cartridge. The resulting weapon was the AR-15, which was adopted by the U.S. Air Force and later by the U.S. Army.

In 1963, the AR-15 was officially adopted by the U.S. military and was designated the M16 rifle. The M16 was first used in combat by U.S. troops in Vietnam in 1965, and it quickly gained a reputation for being a reliable and accurate weapon.

However, the early versions of the M16 were plagued with problems. In particular, soldiers in Vietnam complained that the rifle was prone to jamming, especially in humid conditions. The military eventually addressed these issues by making a number of design changes, including adding a chrome-lined barrel and making improvements to the weapon's magazines and ammunition.

Despite these early problems, the M16 has become one of the most widely-used rifles in the world, and it has been used by numerous military forces in conflicts around the globe. Today, the M16 has largely been replaced by the M4 carbine, a shortened version of the rifle that is better suited for close-quarters combat.