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The AR-15 (for
Armalite model 15) is a widely owned
semi-automatic rifle derived from the
selective fire M16-series
assault rifle, in use by the United States military.
The AR-15 consists of separate upper and
lower receiver assemblies, which are attached with two through-pins and can be quickly interchanged with no tools. The lower receiver (because it bears the weapon's serial number and fire control group) is itself regulated as a firearm. However, the upper receiver assembly is simply considered a part, and may be purchased and mail-ordered in most locations with no restrictions. This is an attractive feature for users because it allows a number of upper receivers (often in different calibers) to be interchanged with the same lower receiver. However, one must be thoroughly familiar with firearms laws before doing this as it is possible to make an
illegal configuration.
Standard AR-15 rifles accept detachable magazines of widely varying capacities, and have a pistol grip that protrudes beneath the stock. AR-15 rifles are highly configurable and customizable. They are commonly fitted with several accessories such as bipods, folding or collapsing stocks, threaded barrels for the attachment of a flash suppressor, and a rail system for the attachment of vertical grips, flashlights, laser sights, telescopic sights and so on.
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. 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. Navy, 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.
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 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 aluminium 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)
Semi-automatic AR-15 for sale to civilians and fully automatic versions for sale to law enforcement and military customers, though nearly identical in appearance, are quite different internally. The hammer and trigger mechanisms are of a slightly different design. The bolt carrier and internal lower receiver of semi-automatic versions are milled differently, so that the firing mechanisms are not interchangeable. This was done specifically to satisfy BATF requirements that civilian weapons may not be easily convertible to fully-automatic. Despite this, through use of a "Drop In Auto Sear" or "lightning-link," conversion to full automatic is very straightforward (sometimes requiring slight modification to the bolt carrier). Such modifications, unless using parts made prior to 1986, are illegal. An illegally converted fully-automatic AR-15 was used in the North Hollywood shootout.
Automatic variants have a three-position 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 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. Due to this, weapons modified to fully automatic using a lightning-link are capable of fully automatic fire only unless a fully-automatic fire select switch is substituted.
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 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 into the AR-15's upper receiver. Here, the gas tube protrudes into a “gas key” (bolt carrier key) which accepts the gas and funnels it into the bolt carrier. The bolt and bolt carrier together form a piston, which is caused to expand as the cavity in the bolt carrier fills with high pressure gas. The bolt is locked into the barrel extension, so this expansion forces the bolt carrier backward in line with the stock of the rifle. As the bolt carrier moves toward the butt of the gun, the bolt cam pin, riding in a slot on the bolt carrier, forces the bolt 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 bolt's rearward motion extracts the empty cartridge case from the chamber, and as soon as the neck of the case clears the barrel extension, the bolt's spring-loaded ejector forces it out the ejection port in the 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 unique extension.
Variants
See also: AR-15 variants
Colt AR-15 Sporter SP1 Carbine
The AR-15 rifle is available in a wide range of configurations from a large 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. Some manufacturers are ArmaLite, Bushmaster, Doublestar and DPMS.
Due to the rifle's modular design, one upper receiver can quickly and easily be substituted for another. There are many aftermarket upper receivers that incorporate barrels of different weights, lengths and calibers. Some available calibers for the AR-15 platform are the .223 Remington/5.56x45mm, 6.5 mm Grendel and 6.8 mm Remington SPC.Care must be taken not to chamber the 5.56x45 NATO into a rifle designated 223 Remington, as it may cause an unsafe high-pressure hazard given that the two calibers are similar, but not identical.
When installing a new complete upper receiver, particularly one designed to handle a different caliber of ammunition (i.e., other than .223 Remington or 5.56x45 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 9 mm typically would involve the installation of a magazine well block (to accommodate a typical 9 mm magazine, such as Uzi or Colt SMG), replacing the .223 hammer with one designed for 9 mm ammunition, and depending on the original stock, replacing the buffer, action spring and stock spacer with those designed for the new 9 mm AR-15 configuration.
Colt AR-15 A3 Tactical Carbine. Rifle is shown with a CQB Tactical Sling and a Colt 4x20 scope.
Early models had a 1:14 rate of twist for the original 55 grain (3.6 g) bullets. This was changed to 1:12 when it was found that 1:14 was insufficient to stabilize a bullet when fired in cold weather. Most recent rifles have a 1:9 or 1:7 twist rate. 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.
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.
