AR-15

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AR-15

AR-15 (for Armalite model 15, often mistaken for Automatic Rifle or Assault Rifle) is the common name for the widely-owned^[1] semi-automatic rifle patterned after the fully automatic M16 and M4 carbine assault rifle, which are currently in use by the United States military. AR-15 was the original name for what became the militarily designated M16, the assault rifle first used by the U.S. in the Vietnam War. The name AR-15 is now used almost exclusively to refer to the semi-automatic (commercially available) civilian version(s) of the M16 and M4 assault rifles.

All standard AR-15 rifles accept detachable magazines of widely varying capacities, and have a pistol grip that protrudes conspicuously beneath the stock. AR-15 rifles are highly configurable and customizable, and are commonly fitted with several accessories, including bipods, bayonet lugs, folding or collapsing butt stocks, threaded barrels for the attachment of a flash suppressor or other accessories, and a Picatinny rail in place of the fore grip for the attachment of vertical grips, flashlights, laser sights, telescopic sights, and other accessories.

History
Operating mechanism
Variants
Legal status

United States

California

Finland

Criticism
See also

Manufacturers

References
External links

History

The AR-15 is based on the 7.62 mm AR-10, designed by Eugene Stoner of the Fairchild ArmaLite corporation.^[2] 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. 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.^[2]

Colt AR-15 Sporter SP1 Carbine

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) which were 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.^[3]^[4]

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:

An AR-15 clone receiver manufactured by DPMS.

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
Synthetic pistol grip (regulated in some jurisdictions)

AR-15 sight picture

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. The pistol grip that protrudes conspicuously beneath the stock was also considered an accessory feature under the 1994 Federal Assault Weapons Ban, and was subject to restrictions. Some AR-15 were manufactured with a grip not described under the Ban installed in its place.

Operating mechanism

Diagram of M16 rifle firing

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 above the rifle's front sight base. The gas rushes into the port and down a gas tube, located above the barrel, which runs from the front sight base to 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. This movement of gas into the bolt carrier forces the bolt and carrier backward in line with the stock of the rifle. As the bolt carrier moves toward the butt of the gun, the bolt begins to turn and unlock from the barrel extension. Once the bolt is fully unlocked it begins its 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 toward the butt of the gun and the empty case is extracted and ejected out the ejection port on side of the upper receiver.

Behind the bolt carrier is a plastic or metal buffer which rests in line with a return spring that pushes the bolt carrier back toward 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 from the magazine which is guided by feed ramps 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

Colt AR-15 A3 Tactical Carbine. Rifle is shown with a CQB Tactical Sling and a Colt 4x20 scope.

The AR-15 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.

There are many aftermarket upper receivers that incorporate barrels of different weights, lengths and ammunition calibers available for the AR-15. Due to the rifle's modular design, one upper receiver can quickly and easily be substituted for another. The different calibers available for the AR-15 platform include, by order of size:

Pistol calibers:

Rifle calibers:

+ In addition to many obscure "wildcat" rifle cartridges.

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 magazine well 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 the original stock, replacing the buffer, action spring and stock spacer with those designed for the new 9mm AR-15 configuration.

Early 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 was shown to be unstable at lower 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, such as 1:10 for 6.8x43mm SPC and 7.62x39mm, and 1:12 for .308 Winchester.

STANAG magazine compatible with the AR-15. Can be used in several other firearms such as the FN F2000 and the M16

Standard issue magazines are 20 or 30 round staggered-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

United States

A California-legal AR-15 clone (FAR-15) with a 10 round magazine. Other notable features include fixed muzzle, forward assist, "bullet button" collapsible stock & M68 (sight optic) mounted on the top rail.

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. Included in this was a restriction on the pistol grip that protrudes conspicuously beneath the stock, which was considered an accessory feature under the ban and was subject to restrictions. Some rifles were manufactured with a grip not described under the Ban installed in its place. Those AR-15's 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,^[5] these features are now legal in most states.^[6]

California

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.^[7] 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, Spikes Tactical, 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.

Finland

In Finland possession of military-style semiautomatic rifles including the AR-15 is legal to volunteers active in military training sponsored the National Defence Training Association of Finland.

Criticism

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. The most common result of firing military 5.56x45mm ammunition in a .223 Remington chamber is that the primer can be forced out of the case by chamber pressure, often resulting in the primer becoming lodged somewhere in the action of the rifle, and disassembly of the rifle is often necessary to remove the jammed primer.^[8] 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 primer of the chambered round. 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.^[9]