AR-15 General Information
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The AR-15 is a lightweight, air-cooled, magazine-fed, autoloading centerfire rifle. The original Armalite/Colt AR-15 was a selective-fire prototype submitted for consideration as a military infantry rifle, and is distinguished from later civilian-model AR-15 and AR-15A2 rifles marketed by Colt Firearms.
The AR-15 is based on the 7.62mm AR-10 by Eugene Stoner of the FairchildArmaLite corporation. The AR-15 was developed as a lighter, 5.56mm caliber version of the AR-10. (The "AR" in AR-15 comes from the Armalite name and does not stand for 'assault rifle' as is commonly believed.)
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, coming in at 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.)
Some revolutionary or otherwise notable features of the 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.
Standard issue magazines are 20 or 30 round double column magazines; drum magazines in 90 and 100 round capacities also exist, such as Beta C-Mags. Low-capacity magazines are available to comply with some areas' legal restrictions, hunting and because larger magazines can inhibit shooting from a benchrest.
The mechanism of operation for the rifle is known as gas impingement. The gas is tapped from the barrel as the bullet moves past a gas port located under the rifle's front sight base. The gas rushes into the port and down a gas tube located above the barrel. The gas tube telescopes over a “gas key” which accepts the gas and funnels it into the bolt carrier. At this point, the gas expanding inside the bolt carrier forces the bolt and carrier in opposite directions. (It operates similar to a piston in reverse.) As the bolt carrier moves towards the butt of the gun, the bolt begins to turn and unlock from the barrel extension. The cam pin is responsible for the bolt's rotation as it follows a groove cut into the carrier that twists and permits the bolt to unlock. Once the bolt is unlocked, the carrier continues to move backwards towards the butt of the gun and the chambered casing is extracted.
A return spring located behind a buffer then pushes the bolt carrier back towards the chamber. A groove machined into the upper receiver traps the cam pin and prevents it and the bolt from rotating into a closed position. The bolt's locking lugs then push a fresh round up the feed ramps and into the chamber. As the bolt's locking lugs move past the receiver extension, the cam pin is allowed to twist into a pocket milled into the upper receiver. This twisting action follows the groove cut into the carrier and permits the bolt to twist and “lock” into the barrel’s extension.