AR-15

The AR-15 is a lightweight, air-cooled, magazine fed, autoloading, centerfire shoulder-fired rifle. The original ArmaLite/Colt AR-15 was a selective-fire prototype submitted for consideration as a military infantry rifle, which was later adopted as the M16, and is distinguished from later civilian-model AR-15 rifles marketed by Colt Firearms. Currently, AR-15 is a generic term for a civilian semi-automatic rifle similar to the military M16/M4-type weapons.

History
The AR-15 is based on the 7.62 mm AR-10, designed by Eugene Stoner of the Fairchild ArmaLite corporation. The AR-15 was developed as a lighter, 5.56 mm version of the AR-10. The "AR" in AR-15 comes from the ArmaLite name, not "assault rifle" as is commonly believed; ArmaLite's AR-1, AR-5, and some subsequent models were bolt action rifles, and there are shotguns and pistols whose model numbers also include the "AR" prefix.

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 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.

Some notable features of the AR-15 include:


 * Aircraft grade aluminum receiver
 * Modular design allows for a variety of accessories and makes repair easier
 * Small caliber, accurate, high velocity round
 * Synthetic stock and grips do not warp or splinter
 * Front sight adjustable for elevation
 * Rear sight adjustable for windage and elevation
 * Wide array of optical devices available in addition to or as replacements of iron sights
 * A direct impingement gas system

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. Civilian AR-15 models usually do not have three-round burst or automatic settings on the fire selector. In semi-automatic only variants, the selector only rotates between safe and semi-automatic. 

Operating mechanism
The mechanism of operation for the rifle is known as direct gas impingement. 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 runs from the front sight base into the AR-15's upper receiver. Here, the gas tube telescopes into a “gas key” which accepts the gas and funnels it into the bolt carrier. The movement of gas into the bolt carrier forces the bolt and carrier backwards in a line with the stock of the rifle. As the bolt carrier moves towards the butt of the gun, the bolt begins to turn and unlock from the barrel extension. Once the bolt is fully unlocked it begins rearward movement along with the bolt carrier. The cam pin is responsible for the bolt's rotation as it follows a groove cut into the carrier that twists and forces the bolt to unlock. Once the bolt is unlocked, the bolt carrier and bolt continue to move towards the butt of the gun and the chambered casing is extracted and ejected out the side of the upper receiver.

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 out of the magazine, up the feed ramps and into the chamber. As the bolt's locking lugs move past the barrel 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 forces the bolt to twist and “lock” into the barrel’s extension. 

Variants
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. These calibers include (in caliber size order):

Pistol calibers:
 * 9mm luger
 * 10mm
 * 40 S&W
 * 45 ACP

Rifle calibers:
 * .22 Long Rifle
 * .204 Ruger
 * 5.56x45mm NATO
 * .223 Remington
 * .223 WSSM
 * .223 Wylde
 * .243 WSSM
 * 6mmAR
 * 6mm-223 Remington
 * 6.5 Creedmoor
 * 6.5 Grendel (.264 cal)
 * 6.8 SPC (.270 cal)
 * .300 Whisper
 * 7.62x39mm
 * .308 Winchester
 * .450 Bushmaster
 * .458 SOCOM
 * .50 AE
 * .50 Beowulf
 * .50 BMG

When installing a new complete upper receiver, particularly one designed to handle a different caliber of ammunition (i.e. other than .223 Remington or 5.56 x 45 mm NATO), some modification to the contents of the lower receiver may also be required, depending on the particular conversion. For example, a conversion to 9mm typically would involve the installation of a magwell block (to accommodate a typical 9mm magazine, such as Uzi or Colt SMG), replacing the .223 hammer with one designed for 9mm ammunition, and depending on your original stock, replacing the buffer, action spring and stock spacer with those designed for your new 9mm AR-15 configuration.

Earliest models had a 1:14 rate of twist, which was changed to 1:12 for original 55 grain (3.6 g) bullets. The 1:14 rate of twist showed to be unstable in colder temperatures. Most newer configurations use 1:9 and 1:7 twist rates. There is much controversy and speculation as to how differing twist rates affect ballistics and terminal performance with varying loads, but heavier projectiles tend to perform better with faster rifling rates. Additionally, the various non .223 / 5.56 calibers have their own particular twist rate.

Standard issue magazines are 20 or 30 round double column magazines, traditional box magazines also exist in 40 and 45 round capacities, and usable magazines have been constructed from a variety of materials including steel, aluminum, and high-impact plastics. Drum magazines with 90 and 100 round capacities also exist, such as Beta C-Mags. Low-capacity magazines, usually of a 5 or 10 round capacity, are available to comply with some areas' legal restrictions, hunting and because larger magazines can inhibit shooting from a benchrest.

Legal status in the United States
In the United States, 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 legal in most states.

The 2000 Assault Weapons ban in the state of California sparked a renewed interest in the AR-15 rifle. It is estimated that some 70,000 California Legal AR-15s are in existence in that state. Adding the upper receiver of a standard AR-15 or equivalent with an AR-15 equivalent lower receiver which has not been specifically banned by statute or regulation, and that has a fixed 10 round magazine will render the firearm "California legal." In such a configuration, the user could add otherwise prohibited features such as a telescoping stock and pistol grip. The magazine is not detachable, so to load the rifle the shooter must pull the rear takedown pin, hinge the upper receiver on the front pivot pin, and load the now exposed magazine either with a stripper clip or by hand, then close. Popular lower receivers for this purpose are manufactured by Stag Arms, Fulton Armory, Dane Armory, Mega, and Ameetec. By California law if the magazine requires a tool to remove it, that changes the classification of the firearm. A tool called the "Bullet Button" is gaining in popularity: the bullet button works by replacing the magazine release button with a hollow shell that protrudes a short distance from the lower; the shooter must then push the inset pin to activate the mag release, doing so requires a tool e.g., a bullet, hence the name. Stag makes a lower receiver called the STAG-15 which is considered an "off-list" receiver by the CA DOJ and is legal. As of December 2006, Doublestar, Stag Arms, CMMG, and MEGA all qualify as "off-list" lowers in the state of CA. There is also one model made by Colt, the CAR-A3 HBAR Elite, that was never banned by name, and thus still legal to own in California provided it has the correct configuration. This receiver can be made into a full rifle if the following requirements are met: the receiver has a fixed magazine with no more than 10 cartridges - in which case the rifle may have pistol grips, folding or collapsing stocks, etc.; or, the receiver may have a detachable magazine but may not possess any sort of attachment such as pistol grips, folding or collapsing stocks, etc.

Concerns
With the plethora of manufacturers of complete weapons and aftermarket barrels, there is a potential hazard associated with chamber specifications. Both civilian (SAAMI) specification .223 Remington and 5.56 mm NATO are available. Though both chambers typically accept both types of ammunition, the firing of military specification ammunition in civilian specification chambers can produce chamber pressures greater than the barrel is designed to handle. Military specification chambers typically have a more open throat area producing less pressure and can handle both types of ammunition.

A few AR15 manufacturers incorporate the use of a hybrid chamber specification known as the Wylde chamber. Designed by and named after Bill Wylde, this chambering was created for High Power shooters after the 80 grain .224" bullets became popular. While the Wylde chamber allows for optimal seating depth of 80 grain bullets over .223 Remington and 5.56 NATO, it is capable of accepting both ammunition types. The Wylde chamber is used by a few manufacturers who sell "National Match" configuration AR-15 rifle, barrels, and upper receivers.

The type of chamber, manufacturer, and rifling twist in inches is typically found stamped into the barrel in front of the front sight assembly.

An additional point of concern in the design is the inertial firing pin. A lightweight firing pin rides in a channel inside the bolt unrestrained. When the bolt locks forward during loading, the firing pin typically rides forward and impacts the chambered round's primer. In military specification ammunition and quality civilian ammunition, this is not normally enough to fire the round and only leaves a small "ding" on the primer. With more sensitive primers or improperly seated primers, this can cause a slamfire during loading.

Manufacturers

 * Alexander Arms
 * ArmaLite
 * Bushmaster
 * Century Arms
 * Charles Daly Defense
 * Colt Manufacturing Company
 * DoubleStar Corp.
 * DPMS Panther Arms
 * DSA
 * Eagle Arms
 * Essential Arms
 * Fabrique Nationale
 * Fulton Armory
 * Gunsmoke Enterprises
 * Heckler & Koch
 * Hesse
 * High Standard
 * JP Enterprises
 * Knight's Armament Company (Stoner)
 * Les Baer Custom
 * Lewis Machine and Tool Co.
 * Olympic Arms
 * PWA
 * Remington Arms
 * Rock River Arms
 * Sendra
 * Smith & Wesson
 * SNS Industries, Inc.
 * Stag Arms
 * Sun Devil Manufacturing
 * Vector Arms
 * Wilson Combat