The Colt AR-15 is a product line of magazine-fed, gas-operated, autoloading rifles manufactured by Colt's Manufacturing Company ("Colt") in many configurations. The rifle is a derivative of its predecessor, the lightweight ArmaLite AR-15, an automatic rifle designed by Eugene Stoner and other engineers at ArmaLite in 1956.

Colt currently owns the AR-15 trademark and uses it for its line of semi-automatic AR-15 rifles.

Due to financial problems, and limitations in terms of manpower and production capacity, ArmaLite sold the AR-15 design and the AR-15 trademark along with the ArmaLite AR-10 to Colt's Manufacturing Company in 1959. Some of the first sales were of fully automatic Colt AR-15s to the Federation of Malaya and the United Kingdoms in 1959. Colt started selling the semi-automatic version of the M16 rifle as the Colt AR-15 in 1964. The first mass production version was the Colt AR-15 Sporter, in .223 Remington, with a 20-inch barrel, issued with 5-round magazines. Colt has since made many different models of AR-15 rifle and carbine models, including the AR-15, AR-15A2, AR-15A3, AR-15A4, and others.

Sale of new AR-15s in the US was banned by the Federal Assault Weapons Ban from 1994 to 2004. Colt and others continued to sell legally compliant versions during that period. On September 20, 2019, Colt announced that it would no longer produce the AR-15 for private use, due to market saturation. It would still produce the AR-15 for police and military units. By May 2020, changed market conditions encouraged Colt to resume production for sales to private users.

The 1956 U.S. patent 2,951,424 by Eugene Stoner describes the cycling mechanism used in the Armalite AR-10 rifle that is common with rifles that use the Stoner "Direct Impingement" gas operated bolt and carrier system including Colt AR-15s. The bolt carrier acts as a movable cylinder and the bolt itself acts as a piston. This mechanism is often called "direct gas impingement", although that is now considered to be a misconception. The mechanism is now referred to as an "internal gas piston operating system".

Gas is tapped from the barrel as the bullet moves past a gas port located above the rifle's front sight base. The gas expands into the port and down a gas tube, located above the barrel that runs from the front sight base into the AR-15's upper receiver. Here, the gas tube protrudes into a "gas key" (bolt carrier key), which accepts the gas and funnels it into the bolt carrier.

At this point, the bolt is locked into the barrel extension by locking lugs, so the expanding gas forces the bolt carrier backward a short distance. As the bolt carrier moves toward the butt of the gun, the bolt cam pin, riding in a slot on the bolt carrier, forces the bolt to rotate and thus unlocks it from the barrel extension. Once the bolt is fully unlocked, it begins its rearward movement along with the bolt carrier. The bolt's rearward motion extracts the empty cartridge case from the chamber. As soon as the neck of the case clears the barrel extension, the bolt's spring-loaded ejector forces it out the ejection port in the side of the upper receiver.

Behind the bolt carrier is a plastic or metal buffer, which rests in line with a return spring. The buffer spring begins to push the bolt carrier and bolt back toward the chamber once it is compressed sufficiently. A groove machined into the upper receiver guides the bolt cam pin and prevents it and the bolt from rotating into a closed position. The bolt's locking lugs push a fresh round from the magazine as the bolt moves forward. The round is guided by feed ramps into the chamber. As the bolt's locking lugs move past the barrel extension, the cam pin twists into a pocket milled into the upper receiver. This twisting action follows the groove cut into the carrier and forces the bolt to twist and "lock" into the barrel extension.