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The AR-15 is a lightweight, magazine-fed, autoloading centerfire rifle designed by Eugene Stoner of the ArmaLite corporation who developed it as a smaller-caliber version of the AR-10.

History

ArmaLite sold its rights to the AR-10 and AR-15 to Colt in 1959 after which the AR-15 was adopted by the United States military under the designation M16. Colt continued to use the AR-15 trademark for its semi-automatic variants. The "AR" in AR-15 comes from the Armalite name and does not in fact stand for assault rifle as is commonly believed. Today the 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:
* Aircraft grade aluminum receiver
* Modular design allows for a variety of accessories, renders repair easier
* Small caliber, high velocity round
* Synthetic stock and grips do not warp or splinter
* Front ironsight adjustable for elevation
* Rear ironsight adjustable for windage and distance
* Wide array of optical devices available in addition to or as replacements of ironsights

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.

Technical data

An M16 Magazine

* Caliber: .223 Remington, 5.56 x 45 mm NATO (many variants in other calibers are made by various manufacturers)
* Length: 39 in (991 mm)
* Mass: 3.4 kg (7.5 lb)
* Barrel: 20 in (508 mm) standard, 16 in (406 mm) and 14.5 in (368 mm) common
* Rifling: Earliest models had a 1:14 rate of twist, which was changed to 1:12 for original 55 grain (3.6 g) bullets. 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.
* Magazine capacity: 5, 10, 20, 30, 40, 90, 100 (see below)

Standard issue magazines are 20 or 30 round double stack 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.

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. Prices tend to be reasonable. These calibers include:
*.22 Long Rifle
*6.5 Grendel
*6.8 SPC
*9mm Parabellum
*7.62x51mm NATO
*7.62x39mm
*.50 AE
*.50 Beowulf
*.50 BMG
*.45 ACP

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.

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.

Legal status in the United States

In the United States, variants with certain features were prohibited for new 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. Since the expiration of the AWB, however, those features are only illegal in part or in whole in certain states.A recent ban of AR-15s in the state of California sparked a renewed interest in the rifle. It is estimated that some 10,000 California Legal AR-15s are in existence in the state. Replacing the Lower Receiver of a standard AR-15 with one that has a fixed (10 round) magazine (see below for instructions) will render the firearm legal, and able to utilize "scary features" such as a telescoping stock and pistol grip. The magazine is not detachable, and to re-load the rifle you must pull a pin, lift the (now hinged) receiver, exposing the magazine, and load either with a stripper clip or by hand, then close. Popular lower receivers for this purpose are manufactured by Stag Arms, Fulton Armory, Mega and Ameetec. Stag makes a lower receiver called the STAG-15 which is considered an "off-list" receiver by the CA DOJ and is temporarily legal. 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 (such as Rock River Arms) incorporate the use of a hybrid chamber specification known as the "Wylde" chamber which provides for SAAMI-like accuracy while allowing for the safe use of military specification ammunition (5.56 NATO). 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.

As in many small caliber firearms, it is possible to rupture the barrel of an AR15 if it is fired with water in the bore. After submersion or if it is suspected water may be in the barrel, it is recommended that the weapon be unloaded, pointed down, and the charging handle pulled to the rear to allow it to drain.

External links

* Modern Firearms article
* ar15.com, a firearms dicussion forum
* Eugene Stoner AR15
* Terminal Ballistics of AR15

Manufacturers

* ArmaLite
* Bushmaster
* Colt
* DPMS
* Lewis Machine & Tool "LMT"
* Olympic Arms
* Rock River Arms
* Tactics LLC

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