The Colt AR-15 is a lightweight, magazine-fed, gas-operated semi-automatic rifle. It is a semi-automatic version of the M16 rifle sold for the civilian and law enforcement markets in the United States. The AR in AR-15 stands for ArmaLite rifle, after the company that developed it in the 1950s. Colt's Manufacturing Company currently owns the AR-15 trademark, which is used exclusively for its line of semi-automatic AR-15 rifles.
Complete AR-15A2 rifle set with manual, 5.56 mm ammunition and magazines
AR-15A2 with the upper and lower receiver opened at the front hinge
The AR-15A2's most distinctive ergonomic feature is the carrying handle and rear sight assembly on top of the receiver.
Gas-operation is a system of operation used to provide energy to operate locked breech, autoloading firearms. In gas-operation, a portion of high-pressure gas from the cartridge being fired is used to power a mechanism to dispose of the spent case and insert a new cartridge into the chamber. Energy from the gas is harnessed through either a port in the barrel or a trap at the muzzle. This high-pressure gas impinges on a surface such as a piston head to provide motion for unlocking of the action, extraction of the spent case, ejection, cocking of the hammer or striker, chambering of a fresh cartridge, and locking of the action.
Long stroke gas piston, from an AK-74
Short stroke gas piston and bolt carrier group, from a gas piston AR-15