The AR-15 is a lightweight, 5.56 mm, magazine-fed, semi-automatic rifle, with a rotating-lock bolt, actuated by direct impingement gas operation or long/short stroke piston operation. It is manufactured with the extensive use of aluminum alloys and synthetic materials.
The AR-15 was first built by ArmaLite as an assault rifle for the United States armed forces. Because of financial problems, ArmaLite sold the AR-15 design to Colt. The select-fire version of the AR-15 entered the U.S. military system as the M16 rifle. Colt then started selling the semi-automatic version of the M16 rifle as the Colt AR-15 for civilian sales in 1963. Although the name “AR-15″ remains a Colt registered trademark, variants of the firearm are independently made, modified and sold under various names by multiple manufacturers.
The AR-15 is based on the 7.62 mm AR-10, designed by Eugene Stoner, Robert Fremont, and L. James Sullivan 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 and stands for “ArmaLite Rifle”. ArmaLite’s AR-1, AR-5, and some subsequent models were bolt action rifles, the AR-7 a semi-automatic rifle and there are shotguns and pistols whose model numbers include the “AR” prefix.
ArmaLite sold its rights to the AR-10 and AR-15 to Colt in 1959. After a tour by Colt of the Far East, the first sale of AR-15s was made to Malaysia on September 30, 1959, with Colt’s manufacture of their first 300 AR-15s in December 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. 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. Later heavy-barrel versions of the civilian AR-15 can weigh upwards of 8.5 lb.
Today the AR-15 and its variations are manufactured by many companies and are popular among civilian shooters and law enforcement forces around the world due to their accuracy and modularity (for more history on the development and evolution of the AR-15 and derivatives see M16 rifle).
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 AR-15 manufacturers make AR-15 clones marketed under separate designations, although colloquially these are sometimes referred to by the term AR-15.
Some notable features of the AR-15 include:
- Aircraft grade forged 7075-T6 aluminum receiver is lightweight, highly corrosion-resistant, and machinable.
- Modular design allows the use of numerous accessories such as after market sights, vertical forward grips, lighting systems, night vision devices, laser-targeting devices, muzzle brakes/flash hiders, sound suppressors, bipods, etc., and makes repair easier.
- Straight-line stock design eliminates the fulcrum created by traditional bent stocks, reducing muzzle climb.
- Small caliber, accurate, lightweight, high-velocity round (.223/5.56x45mm).
- Easily adapted to fire numerous other rounds.
- Front sight adjustable for elevation.
- Rear sight adjustable for windage (most models) and elevation (some models).
- Wide array of optical aiming devices available in addition to or as replacements of iron sights.
- Direct impingement gas system (as designed) with short or long stroke gas piston, or direct blowback operating systems available.
- Synthetic pistol grip and butt stock that do not swell or splinter in adverse conditions (regulated in some states).
- Various magazine capacity, ranging from 10 to 30-round or more.
- Ergonomic design that makes the charging handle, selector switch (safety), magazine release, and bolt catch assembly easy to access.
- 4 MOA Accuracy as a MILSPEC standard.
AR-15 sight picture
Semi-automatic AR-15s for sale to civilians are internally different from the full automatic M16, although nearly identical in external appearance. The hammer and trigger mechanisms are of a 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 to satisfy United States Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) requirements that civilian weapons may not be easily convertible to full-automatic. In the late 1970s and early ’80s, items such as the “Drop In Auto Sear” or “lightning-link,” conversion to full automatic was very straightforward (sometimes requiring machining of the lower receiver with use of a lathe and M16 Bolt Carrier Group). Such modifications, unless using registered and transferable parts made prior to May 19, 1986, are illegal. (The Firearm Owners Protection Act in 1986 has redefined a machine gun to include individual components where a semi-automatic firearm can be converted to full-automatic based on a 1981 ATF ruling on machine gun parts.) Since 1993, The Bolt Carrier Groups used in AR-15 type rifles for civilians have employed additional measures to prevent modification to full auto. Colt AR-15’s use a metal alloy wall separating the Fire Control group from the sear, preventing use of such items.
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 Colt AR-15 models do not have three-round burst or automatic settings on the fire selector, though some other makers may mark that way for collectors and re-enactors, even though the guns will not fire in that mode. In semi-automatic only variants, the selector only rotates between safe and semi-automatic. Due to this, weapons modified to full automatic using a lightning-link are capable of full automatic fire only—unless a special full automatic fire select mechanism and modified selector-switch is substituted.
The main 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 a short distance in line with the stock of the rifle to first unlock the bolt. 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. (The gas system only serves to unlock the bolt once the projectile has exited the barrel). 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. The bolt is much heavier than the projectile, and along with the recoil-spring pressure inside the stock buffer-tube performs the cartridge ejection function and chambers the following cartridge.
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.
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.
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, .45 ACP, 5.7x28mm, 6.5 mm Grendel, .338 Lapua, 6.8 mm Remington SPC, .50 Beowulf, .50 BMG, and .458 SOCOM. It is not recommended to chamber the 5.56×45 NATO into a rifle designated .223 Remington, due to the increased chamber pressure in the 5.56mm cartridges; 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 5.56×45 mm NATO), some modification to the contents of the lower receiver may 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. The 9mm cartridge fires from an unlocked breech, or straight blow-back—rather than a locked breech, because the spring and bolt provide enough weight to allow this type of functioning. These guns do not utilize the direct gas impingement method of operation like the original.
Some AR-15’s like the POF, LWRCI, H&K, Sturm Ruger, Sig Sauer, and Adams arms offerings replace the DGI (direct gas impingement) operating system with a short stroke/long stroke gas piston system. These guns usually have modified bolt carriers, gas keys, and gas blocks. When fired, DGI systems dump high pressure hot gas through the gas tube to the bolt carrier key and into the bolt carrier group. This can rapidly heat up the bolt carrier group and cause excessive fouling, one of the main complaints about the design. Gas piston operating systems alleviate these problems, but can be the cause of other issues such as bolt carrier tilt.
Some manufacturers offer upper and lower receivers machined from a solid billet (block) of aluminum as opposed to an aluminum forging. These include Sun Devil manufacturing, LAR Grizzly manufacturing, POF-USA, and Black Rain. This is usually done for added strength.
Upper receivers utilizing a monolithic rail system that combine a railed hand guard and upper receiver into one uninterrupted piece are made by companies like Colt’s Manufacturing Company, Lewis Machine and Tool (LMT MRP), POF-USA, and VLTOR. This is done to provide a continuous uninterrupted rail section that runs along the top of the gun from the weapons charging handle to the front sight/gas block. This rail section is used for the mounting of sights, laser aiming devices, night vision devices, and lighting systems.
A side charging upper receiver has been developed by LAR Grizzly. The charging handle can be had in a left side, right side, or ambidextrous configuration. Since the charging handle is attached to the bolt carrier making it a reciprocating design, it can be utilized as a forward assist device as well.
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, 1/11 and 1/12 for 6.8x43mm SPC, 1/10 7.62x39mm, and 1:12 for .308 Winchester.
Standard issue magazines are 20 or 30 round staggered-column magazines, traditional box magazines 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 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. Surefire is now offering extended capacity magazines in 60- and 100-round capacity configurations. These magazines are a staggered column design.