The AR-15 comes in many sizes and has many options, depending on the manufacturer. The part shown bottom center is the lower receiver with pistol grip and trigger assembly.
|Place of origin||United States|
|Designer||Eugene Stoner, Jim Sullivan, Bob Fremont|
|Manufacturer||ArmaLite, Colt, Bushmaster, Rock River Arms, Stag Arms, DPMS Panther Arms, Smith & Wesson, Ruger, Anderson, Daniel Defense, CMMG, Aero Precision, Olympic Arms and others.|
|Weight||2.27 kg–3.9 kg (5.5–8.5 lb)|
|Cartridge||5.56×45mm NATO and others; see list of AR platform calibers|
|Action||Direct impingement or Gas Piston / Via a Rotating bolt|
|Effective firing range||~550 metres (600 yd)|
|Feed system||Detachable or integral magazine, 5.56mm / .223 Remington versions use STANAG magazines
Belt feed in some versions
|Sights||Adjustable front and rear iron sights|
The AR-15 is a lightweight, intermediate cartridge magazine-fed, air-cooled rifle with a rotating lock bolt, actuated by direct impingement gas operation or long/short stroke piston operation. It has been produced in many different configurations, including semi-automatic. It employs aluminum alloys and synthetic materials.
The prototype AR-15 rifle was designed by ArmaLite as a selective fire weapon for military purposes. Armalite sold the design to Colt due to financial difficulties. After some modifications, the rifle eventually became the US Army's M16 rifle.
The term "AR-15" signifies "Armalite rifle, design 15". Today, Colt uses "AR-15" for its semi-automatic civilian rifles and thus many use the term only for Colt AR-15s and clones made by other manufacturers. This article discusses the original design intended for military users and its major variants.
The trademark "AR15" or "AR-15" is registered to Colt, which requires the term to be used only to refer to their products. Other manufacturers make AR-15 clones marketed under separate designations, although colloquially these are sometimes referred to by the term AR-15.
AR-15 rifles are lightweight, gas-operated, magazine-fed and air-cooled. They fire an intermediate cartridge and are manufactured with extensive use of aluminum alloys and synthetic materials. The design splits the rifle into two major components: the lower half, containing the trigger and buttstock and the upper half, which contains the bolt and barrel. This approach allows modular replacement of components.
- 1 History
- 2 Notable features
- 3 Civilian and military models
- 4 Operating mechanism
- 5 Variants
- 6 Reliability
- 7 See also
- 8 References
- 9 Bibliography
(For more history on the development and evolution of the AR-15 and derivatives, see M16 rifle.)
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 all ArmaLite pattern firearms simply stands for "ArmaLite Rifle" and can be found on most of the company's firearms: AR-5, a .22 caliber rifle; the AR-7, another .22 caliber; the AR-17 shotgun; the AR-10 rifle; and the AR-24 pistol.
In 1959, ArmaLite sold its rights to the AR-10 and AR-15 to Colt. After a tour by Colt of the Far East, the first sale of AR-15s was made to Malaya on September 30, 1959. Colt manufactured their first 300 AR-15s in December 1959. Colt marketed the AR-15 rifle to various military services around the world. After modifications (most notably the relocation of the charging handle from under the carrying handle to the rear of the receiver), the redesigned rifle was adopted by the United States military as the M16 rifle.
In 1963, Colt started selling the semi-automatic version of the M16 rifle as the Colt AR-15 for civilian use and the term has been used to refer to semiautomatic-only versions of the rifle since then. Colt continued to use the AR-15 trademark for its semi-automatic variants (AR-15, AR-15A2) that 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.
The 1986 Firearm Owners Protection Act (FOPA) redefined the term machine gun to include individual gun components with which a semi-automatic firearm can be converted to full-automatic, based on a 1981 ATF ruling on machine gun parts.
In 1993, the bolt carrier groups used in civilian AR-15 type rifles began to employ additional measures to prevent modification to full auto.
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.
Some notable features of the AR-15 include:
- Can be made from forged 6065, and aircraft-grade billet 7075-T6 aluminum receiver that is lightweight, highly corrosion-resistant and machinable.
- Modular design that 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, and numerous variation of aftermarket buttstocks and handguards etc., the ease of parts replacement makes repair easier.
- Straight-line stock design that eliminates the fulcrum created by traditional bent stocks, reducing muzzle climb.
- Interchangeable uppers with different types of calibers. Calibers range from the normal .223/556 cartridge to the 300 Blackout, 6.8 Special, 30 RAR, and larger calibers such as the .450 Bushmaster, .458 SOCOM, and the .50 Beowulf.
- Front sight that is adjustable for elevation
- Rear sight that is 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
- Stoner gas impingement system (as designed), or short or long stroke gas piston, or direct blowback operating systems available
- Synthetic pistol grip and butt stock that do not swell or splinter (regulated in some states)
- Multiple magazine capacities, ranging from 10 to 30-round or more
- Ergonomic design that makes the charging handle, selector switch (which also engages the safety), magazine release and bolt catch assembly easy to access. Some models have ambidextrous controls.
- 4 MOA accuracy or better
Civilian and military models
After the Colt Armalite AR-15 rifle was purchased by the US military it was released to the public as a semi-automatic rifle The first military version was the M-16, a selective fire full or semi automatic rifle. The M-16 has evolved through several variations. It has largely been replaced by a carbine version designated M-4.
The current M-16 and the current M-4 have a three-position rotating selective fire switch, allowing the operator to select between three modes: safe, semi-automatic and three-round burst. A new model of the M-4, the M-4A1 will have fully automatic capability. 
Post-1986 civilian AR-15s support only semi-automatic fire (one shot per trigger pull). They do not have three-round burst or automatic settings; they can only operate as a semi-automatic and are therefore not selective fire weapons (see Modified Civilian below for legally owned full auto AR-15's). Some civilian variants like the Troy PAR are completely manually operated
The original design of the Colt mode switch toggles between safe (safety on) and semi-automatic modes, usually marked "SAFE" and "FIRE." Some other manufacturers may mark their rifles with full automatic and three-round mode positions (for collectors and re-enactors) although these settings are for show and have no effect on the function of the rifle.
Civilian models are internally different from the M16 and M4, although nearly identical in external appearance. The hammer and trigger mechanisms are of a different design. The firing mechanism (bolt carrier and internal lower receiver) of semi-automatic versions is milled differently, so that they are not interchangeable with M16's or M4's. The design was intended to satisfy United States Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) requirements that civilian weapons not be easily convertible to full-automatic.
Civilian models can no longer be legally modified to full automatic in the United States. Today, the civilian manufacture, sale and possession of post-1986 select-fire AR-15 variants or automatic trigger group components is prohibited, per the Hughes Amendment to the Firearm Owners Protection Act. However, it is legal to sell templates, tooling and manuals to conduct such conversion. These items are typically marketed as "post-sample" materials for Federal Firearm Licensees. They may be used to manufacture select-fire variants of the AR-15 for sale to law enforcement, military and overseas customers.
In the late 1970s and early 1980s, conversion to full automatic was straightforward, using items such as the "Drop In Auto Sear" or "lightning link". In some cases such conversion required machining the lower receiver with a mill, as well as the substitution of a M16 bolt carrier group. Modified weapons did not thereby become able to switch between the three modes of the military model. The latter required a special full automatic fire select mechanism and a modified selector-switch. Many AR-15's made before 1986 were converted to M16s by gunsmiths who legally, producing Form One rifles. A converted AR has an auto sear in a lower receiver marked as an AR-15.
FOPA redefined the definition of a machine gun to include individual components with which 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 to separate the fire control group from the sear, preventing use of full automatic parts.
U.S. Patent 2,951,424 describes the cycling mechanism used in the AR-15. The bolt carrier acts as a movable cylinder and the bolt itself acts as a stationary piston. This mechanism is often called "direct gas impingement" (DGI), although it differs from prior gas systems.
Gas is tapped from the barrel as the bullet moves past a gas port located above the rifle's front sight base. The gas expands into the port and down a gas tube, located above the barrel that 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.
At this point, the bolt is locked into the barrel extension by locking lugs, so the expanding gas forces the bolt carrier backward a short distance. 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 rotate and thus unlocks it 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. 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. The buffer spring begins to push the bolt carrier and bolt back toward the chamber once it is compressed sufficiently. A groove machined into the upper receiver guides the bolt cam pin and prevents it and the bolt from rotating into a closed position. The bolt's locking lugs push a fresh round from the magazine as the bolt moves forward. The round is guided by feed ramps into the chamber. As the bolt's locking lugs move past the barrel extension, the cam pin twists 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 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.
The AR-15 employs a modular design. Thus one upper receiver can quickly and easily be substituted for another from the aftermarket. Many aftermarket upper receivers are available that incorporate barrels of different weights, lengths and calibers. Available calibers include the .223 Remington, 5.56×45mm NATO, .300 Blackout, 7.62×39mm, 5.45×39mm, .45 ACP, 5.7×28mm, 6.5mm Grendel, 6.8mm Remington SPC, .50 Beowulf and .458 SOCOM.
When installing a complete upper receiver, particularly one designed to handle a different caliber, the lower receiver may also require modification. For example, conversion to 9 mm ammunition typically typically involves the installation of a magazine well block (to accommodate a typical 9 mm magazine, such as Uzi or Colt SMG), replacing the .hammer and possibly replacing the buffer, action spring and stock spacer with 9mm-compatible components. 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 it. 9mm guns do not use direct gas impingement.
Some manufacturers offer upper and lower receivers machined from a solid billet (block) of aluminum as opposed to an aluminum forging. Forgings typically have a comparatively higher strength to weight ratio than billet-based receivers.
Upper receivers that combine a railed hand guard and upper receiver into one unit are made by companies like Colt's Manufacturing Company, Lewis Machine and Tool (LMT MRP), POF-USA and VLTOR. This provides a continuous rail section that runs along the top of the gun from the weapon's 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.
LAR Grizzly and Blackwood Arms offer a side charging upper receiver. The charging handle can be had in a left side, right side or ambidextrous configuration. The side charging handle is attached to the bolt carrier, making it a reciprocating design. The handle can be used as a forward assist device.
There is a type of .223 barrel chambering which is being used by many manufactuers now to increase the accuracy of AR-15 type rifles which are chambered in 5.56mm Nato. The specification is called .223 Wylde. The Wylde chambering is considered a MATCH accuracy barrel.
AR-15s such as the POF, LWRCI, H&K, Sturm Ruger, SIG Sauer, United Defense Manufacturing Corporation, CMMG and Adams Arms offerings replace the DGI 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 send high pressure hot gas through the gas tube to the bolt carrier key and into the bolt carrier group. This can rapidly heat the bolt carrier group and cause fouling, one of the main design complaints. Gas piston operating systems alleviate these problems, but can cause issues such as carrier tilt, which can increase bolt fractures.
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 in cold weather 1:14 was insufficient to stabilize the bullet. Most recent models have a 1:9 or 1:7 twist rate. The degree to which differing twist rates affect ballistics and terminal performance given varying loads is controversial, although heavier, longer projectiles tend to perform better with faster rifling rates. Additionally, the various each caliber has its own particular twist rate(s), such as 1:10, 1:11 and 1:12 for 6.8×43mm SPC, 1:10 for 7.62×39mm, 1:9 for the 6.5 Grendel and 1:8 for .300 Blackout.
Most AR-pattern rifles use some type of detachable box magazine. Standard issue STANAG magazines are 20- or 30-round staggered-column boxes, and extended traditional box magazines exist in 40- and 45-round capacities. Larger quad-column "casket" magazines can have higher capacities, with Surefire offering 60- and 100-round versions.
Drum magazines can also be used with the AR magwell, with the popular Beta C-Mag holding 100 rounds of 5.56mm NATO or .223 Remington. Some light machine gun AR variants such as the experimental CMG-1 and Ares Shrike 5.56 use a modified upper designed to accept a belt feed.
Low-capacity magazines, usually of a 5- or 10-round capacity, are available to comply with some areas' legal restrictions, for hunting and for benchrest shooting, where a larger magazine can be inconvenient.
In states with capacity limits on fully detachable magazines, "bullet button" lowers have been designed which require a tool (such as the point of a bullet, hence the name) to detach the magazine from the weapon. Certain variants such as the Olympic Arms OA-96 have been manufactured with fully fixed magazines.
Most AR-15 rifles have a barrel threaded in 1⁄2"-28 threads to incorporate the use of a muzzle device such as a flash suppressor, sound suppressor or muzzle brake. The initial design had three tines or prongs and was prone to breakage and getting entangled in vegetation. The design was later changed to close the end to avoid this problem. Eventually, on the A2 version of the rifle, the bottom port was closed to reduce muzzle climb and prevent dust from rising when the rifle was fired in the prone position. For these reasons, the US military declared this muzzle device a compensator, but it is more commonly known as the "GI", "A2", or "Birdcage" muzzle device.
Flash suppressors are designed to reduce the muzzle flash from the weapon to preserve the shooter's night vision. A flash suppressor does not improve the ballistic performance of a rifle or make it more lethal. Some jurisdictions ban or severely restrict usage of flash suppressors. Muzzle brakes or compensators can provide a legal alternative.
The threaded barrel allows sound suppressors with the same thread pattern to be installed directly on the barrel. This can result in complications such as preventing the removal of the suppressor from the barrel. Some suppressor manufacturers offer "direct-connect" sound suppressors that can be installed over an existing flash suppressor, muzzle brake, or compensator as opposed to using the barrel's threads.
A lightweight inertial 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 (mil-spec) ammunition and quality civilian ammunition, this is not normally enough to fire the round and only leaves a small "ding" on the primer.
Many different types of firing pin designs have been tried to make the AR-15 more reliable, or to ensure longer life of the firing pin. Some companies have made titanium firing pins from various types of titanium, carbon and tool steels, and even aluminum. The most popular and most common firing pins are those made of carbon steel. Titanium firing pins are more expensive, and the aluminum firing pins that were tried, failed.
Titanium firing pins will be stronger and lighter than their carbon steel counterparts. These attributes of titanium can save up to 40% of the weight in a firing pin of identical dimensions which can do many things for the firearm. Since the firing pin is lighter, the lock time of the bolt is less. Since the firing pin is much lighter, the chances of a slam fire are also significantly less. Titanium is also a lot more corrosion resistant than carbon steel, and has a higher variation of working temperatures meaning it can take hotter temperatures and lower ones before it fails compared to steel.
- 5.56×45mm NATO
- 6mm TCU
- 6mm AR
- 6mm AR Turbo 40˚ Improved
- 6mm PPC
- 6mm Whisper
- 6mm WOA
- 6mm Hagar
- 6mm BR Remington
- 6.5mm Whisper
- 6.5mm Grendel
- 6.5mm PPC
- 6.5mm WSSM
- 6.5 WOA
- 6.8mm Remington SPC
- 7mm Whisper
- 7mm TCU
- 7.62×40mm Wilson Tactical
- 7.62×51mm NATO
- .17 Practical
- .17 Remington
- .17 HMR
- .17 HM2
- .17 WSM
- .204 Ruger
- .20 Tactical
- .20 Practical
- .20 VarTarg
- .204 Ruger
- .221 Fireball
- .22 LR
- .22 WMR
- .222 Remington
- .222 Remington Magnum
- .223 Remington
- .223 Remington Ackley Improved
- .223 WSSM
- .243 LBC
- .243 WSSM
- .25 WSSM
- .25-45 Sharps
- .264 LBC
- .277 Wolverine
- .30 Remington AR
- .300 Whisper
- .30 Herrett Rimless Tactical
- 300 AAC Blackout (7.62×35mm)
- .300 OSSM
- .30 Carbine
- .338 Lapua Magnum 
- .35 Gremlin
- .358 WSSM
- .450 Bushmaster
- .458 SOCOM
- .50 Beowulf
American soldiers using M16s in Vietnam reported problems affecting the AR-15's reputation. Some problems were attributed to ammunition. Different propellants were used, including IMR 4475 in United States Army cartridges and WC 846 for Air Force ammunition. Ball propellant WC 846 caused fouling and excessive cycle rates. With unrealistic expectations for the first United States combat use of non-corrosive primers M16s were issued without cleaning kits. In the interest of avoiding training accidents, many soldiers had inadequate opportunities to familiarize themselves with the M16 prior to combat.
The direct gas impingement system sometimes caused rapid overheating of the bolt carrier assembly with combustion residue reducing self-loading reliability after firing as few as (200) rounds. This basic design flaw also renders the AR-15 unreliable except in "range-clean" condition. Sand, dirt, and moisture in the buffer tube region may require manual forward assistance to return the bolt to battery.
Firing military 5.56×45mm NATO cartridges may produce dangerously high pressures in civilian chambers designed for the .223 Remington. Excessive pressure may force the primer out of the case into the rifle action. Disassembly may be required to locate and remove the jammed primer.
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