The AR-15 is based on the 7.62mm AR-10, designed by Eugene Stoner of the Fairchild ArmaLite corporation. The AR-15 was developed as a lighter, 5.56mm version of the AR-10. The "AR" in AR-15 comes from the ArmaLite name.

 ArmaLite sold its rights to the AR-10 and AR-15 to Colt in 1959. Colt marketed the AR-15 rifle to various military services around the world, including the U.S. Air Force, Army, and Marine Corps. The AR-15 was eventually adopted by the United States military under the designation M16. However, Colt continued to use the AR-15 trademark for its semi-automatic variants (AR-15, AR-15A2) marketed to civilian and law-enforcement customers. The original AR-15 was a very lightweight weapon, weighing less than 6 pounds with empty magazine, though later heavy-barrel versions of the civilian AR-15 can weigh upwards of 8.5 lbs.

 Today the civilian-model AR-15 and its variations are manufactured by many companies and have captured the affection of sport shooters and police forces around the world due to their low cost, accuracy, and modularity. (Please refer to the M16 for a more complete history of the development and evolution of the AR-15 and derivatives.)

 The trademark "AR15" or "AR-15" is registered to Colt Industries, which maintains that the term should only be used to refer to their products. Other manufacturers make AR-15 clones marketed under separate designations, although colloquially, these are sometimes also referred to by the term "AR-15".

 Semi-automatic and automatic variants of the AR-15 are effectively identical in appearance. Automatic variants have a rotating selective fire switch, allowing the operator to select between three modes: safe, semi-automatic, and either automatic or three round burst, depending on model. In semi-automatic only variants, the selector only rotates between safe and semi-automatic.

 The AR15 rifle is available in a wide range of configurations from a number of manufacturers. These configurations range from short carbine-length models with features such as adjustable length stocks and optical sights, to heavy barrel models.

 Aftermarket upper receivers that incorporate barrels of different weights and lengths, and handle different caliber ammunition, abound for this rifle. They are very easily installed, due to the rifle's modular design.

M-16 Assault Rifle
 The M16 is a lightweight, 5.56 mm caliber, air-cooled, gas-operated, magazine-fed rifle, with a rotating bolt, actuated by direct impingement gas operation. It is constructed of steel, aluminum and composite plastics.

 M16 is the U.S. military designation for a family of rifles derived from the ArmaLite AR-15 and further developed by Colt.

 The M16 Rifle family including the M16/A1/A2/A3/A4 has been the primary infantry rifle of the United States military since the 1960s, is in use by 15 NATO countries, and has been the most produced firearm in its caliber.

 The M16 was an initial version fielded in the early 1960s; however the U.S. Army began to field the M16A1 in 1965 with priority going to Vietnam. A transitional issue was provided to US Army units stationed in Europe. Nearly all US Army units had switched over to the system by 1967. US Army Basic Training units retained the M-14 rifle up through 1969. The US Marine Corps also adopted the system during this period. Adoption of the M16A2 by both the US Army and USMC started in the late 1980s, and the M16A4 began to be fielded even more recently. They are automatic rifles, firing a 5.56 mm round. During the late 1970s a roughly standardized load for this ammunition was adopted throughout NATO.

 The M16A2 entered service in the 1980s and fired the NATO standard, the Belgian-designed M855/M856 cartridge. The M16A2 is a select fire rifle capable of semi-automatic or three-round bursts and incorporating design elements desired by the Marine Corps. These features included an adjustable rear-sight for windage and elevation, a 5/8 inch longer buttstock, heavier barrel, case deflector for left-handed shooters, and rounded handguards. Mode of fire is determined by using a selector switch on the side of the weapon. The M16A3 was essentially an M16A2 with an M16A1 fire control group; this had only limited procurement by the USN.

 The M16A4 became standard issue for the United States Marine Corps during Operation Iraqi Freedom, increasingly replacing the earlier M16A2. In the United States Army, a combination of the M16A4 and M4 Carbine continue to replace existing M16A2 Rifles. The A4 incorporates a flattop receiver unit developed for the M4 Carbine. The USMC version of the M16A4 features a handguard with four Picatinny rails for mounting optical sights, laser pointers, night vision, forward handgrips, removable carry handle and flashlights.

 The M16 is primarily manufactured by Colt and Fabrique Nationale de Herstal, with variants produced by numerous countries around the world. Semi-automatic versions, generally referred to as "AR-15s" (because of Colt's purchase of the nomenclature from Armalite), are popular recreational firearms in the United States, with versions manufactured by a handful of larger manufacturers and over a dozen smaller concerns.

 The M16 was first adopted in 1964 by the United States Air Force (USAF) as the M16. Various modified versions of the M16 design were subsequently fielded under experimental designations, culminating in the M16A1. The M16A1 was simply the M16 with a forward assist as requested by the Army. This weapon remained the primary infantry rifle of the United States military from 1967 until the 1980s, when it was supplemented by the M16A2. The M16A3 is a fully-automatic variant of the M16A2, issued primarily within the United States Naval Special Warfare. The M16A2, in turn, is currently being supplemented by the M16A4, which incorporates the flattop receiver unit developed for the M4 Carbine. Previous versions of the weapon are still in stock and used primarily by reserve and National Guard units in the United States as well as by the U.S. Air Force.

 In the United States, AR-15 and M-16 variants with certain features such as collapsible stocks, flash suppressors, and bayonet lugs were prohibited for sales to civilians during the period 1994-2004 by the Violent Crime Control and Law Enforcement Act of 1994, under the provision known as the Assault Weapons Ban. Those that were manufactured with those features were stamped, "Restricted Military/Government/Law Enforcement/Export Only" as well as the accompanying high capacity magazines. Since the expiration of the Federal AWB in September of 2004, these features are now illegal only in certain states.