Ar-15 – type semi-automatic Rifles

Special feature article by Maxim Popenker, text © 2012.

Brief overview.

Before we start, it must be pointed out that “AR-15” is a registered trade mark of Colt’s Manufacturing Co, Inc. Therefore, only rifles made by Colt or Colt licensees can be legally marked as “AR-15”. Rifles of similar design but made by everyone else are usually described as “AR-15 type” or “AR-15 style” and bear various proprietary model designations, such as SR-15, XM-15, Z-15 etc.

original "Colt AR-15" logo on the military issue M16A1 rifle

The family of semi-automatic rifles, based on the original Armalite Ar-15 / Colt M16 automatic rifles, is one of the broadest and most popular in the world. Weapons based on same basic design features and same basic specifications, originally established by Colt and approved by US Government (so called Mil-Spec, for “Military specifications”) are manufactured in hundreds of versions and configurations, dozens of calibers and in many countries of the world. Obviously, the USA is the major source and consumer of these rifles, but similar weapons also are manufactured in Canada, Czech republic, China, Germany, Italy, Turkey, Switzerland and even Russia and Ukraine.

History and present.

The AR-15 designation itself comes from the Armalite division of the Fairchild Republic Corporation, which originally developed this rifle during second half of the 1950s as its fifteenth weapon project. Colt bought AR-15 rifle design and manufacturing rights from Armalite in 1959. More details on the origins of the AR-15 military rifles can be found in the M16 rifle article.

Original Ar-15 “Sporter” rifles were manufactured for civilian market by Colt since 1963. These were no more than semi-automatic versions of the original AR-15, M16 and M16A1 rifles, with basically same external appearance and furniture. Initial popularity of these semi-automatic weapons was quite limited. Things began to change gradually in late 1980s and early 1990s, as M16A2 rifle proved itself well as a military weapon, and Colt began to manufacture its semi-automatic only, civilian-legal AR-15A2 “Sporter” and “Match Target” versions. Several other companies soon joined the club, manufacturing rifles based on aforementioned military specifications to assume parts interchangeability.

Colt AR-15 Sporter SP1 semi-automatic rifle, based on the early XM16E1 military rifle

Colt AR-15A2 Match Target Heavy Barrel semi-automatic rifle, based on the M16A2 military rifle

Early Ar-15 style rifles were almost invariably manufactured to fire ammunition of same caliber as its military ancestor, the .223 / 5.56mm. Early caliber alternatives for Ar-15 style rifles introduced in 1980s, were limited to .22LR for low cost training (by using special adapters) and 9x19 and .45ACP pistol ammunition for short-range shooting (including home defense). Latter options required a dedicated 9mm or .45ACP upper receiver with appropriate barrel, bolt group, magazine adapter and magazines.

During last two decades (1990s and 2000s) however, the spectrum of calibers available for Ar-15 style rifles broadened noticeably. Among earliest adoptions was the ex-Soviet 7.62x39, low-cost, mid-range cartridge with sufficient power to hunt soft-skinned medium game and provide good stopping power for home defense. It also required dedicated upper receiver with 7.62mm barrel and bolt, as well as dedicated magazines for reliable functioning. Recent contender to the venerable Russian 7.62mm war-horse is the .300 AAC Blackout / .300 Whisper cartridge, which, although more expensive, is also more accurate and somehow more versatile. It also fits into the standard 5.56mm magazines and bolt faces. Other modern cartridges like 6.5 Grendel or 6.8 Remington SPC offer good compromise between 5.56mm and 7.62mm ammunition for rifles, originally designed for 5.56mm ammunition. Introduction of several specially designed big-bore cartridges, which can be crammed into standard Ar-15 magazines (in single, rather than in double stack), put this rifle into medium and even big game hunting class, although only at short- to medium ranges. Cartridges like .458 SOCOM, .499 LWR or .50 Beowulf fire bullets heavy and powerful enough (usually weighting 26+ gram / 300+ grains with muzzle energy in excess of 3000 Joules / 2200 ft/lbs) to drop down many medium-sized and some big animals at ranges of up to 100-200 meters. Obviously, these cartridges require dedicated upper receivers with appropriately chambered barrels, but most of these rounds are designed with rebated rims to function with standard bolts with 5.56 / .223 bolt faces.

just some of the most popular cartridges that can be used in AR-15 style rifles

Colt Sporter carbine in 9x19 Luger

Barret M468 carbine in 6.8 Remington SPC

Zbroyar Z-15 carbine in 7.62х39

Alexander Arms carbine in .50 Beowulf

Needless to say, users are not limited to choice of calibers only. Even with the basic 5.56 / .223 chambering, one can fit his (or hers) lower receiver with uppers equipped with barrels 16” to 24” long (shorter barrel length are available in USA under limitations of National Firearms Act of 1934, laws in other countries are different), with various profiles (from ”light carbine” to “heavy match target”), furniture etc.

Pros and cons of the Ar-15

Like any other system, Ar-15 has its own positive and negative features. Among positive features of the basic Ar-15 style rifles are excellent modularity, wide availability of spare and replacement parts and accessories, good potential for accuracy, and good ergonomics.

Established specifications for part-to-part interfaces, combined with original modular construction of the Ar-15 receiver result in excellent flexibility of design. Anyone can easily swap various upper receivers on a single lower receiver (which, per US laws, is the only registered and serially numbered part of the weapon), changing barrel lengths and profiles, sighting options, calibers etc. Wide availability of spare and aftermarket parts allows to customize the AR-15-style rifle for almost any purpose and budget – low-cost plinker, home-defense gun, hunting rifle (for small and medium game, from varmint to deer and hogs), high-end target or IPSC or 3-Gun rifle – you name it, you can buy it, or build it yourself, or have it built for you by a gunsmith or custom manufacturing company.

Bushmaster XM-15-E2S carbine

AR-15 type pistol

JP SCR-11 high-end match rifle

Remington R-15 VTR hunting rifle

Stag Arms Model 7L left-hand rifle

Smith&Wesson MP-15 FT carbine

Properly designed and built Ar-15 style rifles also can be quite reliable (firing thousands of rounds without single malfunction) and accurate (match or varmint versions of Ar-15 can repeatedly shoot sub-MOA groups with properly selected ammunition). Good ergonomics of the basic platform can be further enhanced by installation of fully ambidextrous controls, furniture of various shapes and dimensions, Picatinny rail interfaces etc.

Negative features of typical AR-15 style rifles include sensitivity to ammunition and relatively high maintenance requirements, at least when compared to its long-time rivals, Kalashnikov AK-style rifles. Another weak point of the AR-15 is flimsy design of the original magazines. Yet another feature which can be considered as a negative of a sort is the buffer tube housing, which protrudes rearwards from the receiver and into the shoulder stock, precluding usage of the folding stocks.

It must be noted, however, that most negative features are addressed to a certain degree by recent improvements, such as piston-operated conversions, high quality after-marked magazines etc.

Design features

The basic civilian Ar-15 type rifle is a semi-automatic, gas operated weapon. It fires only one cartridge per every trigger pull, and therefore it cannot be properly considered to be an “Assault rifle”, which, by a common definition, is a select-fire (full automatic or burst-firing) weapon.

Standard Ar-15 action, as designed by Eugene Stoner, is powered by hot powder gases, which are fed from the bore via gas tube and into the bolt carrier. Inside the bolt carrier there is a variable volume cavity, formed by the bolt carrier and bolt. Hot gases expand there, pushing the bolt forward against the pressure of the cartridge base, and simultaneously pushing the bolt carrier rearwards. The rearward movement of the bolt carrier forces the bolt to rotate via cam and pin setup. This rotation disengages seven radial locking lugs, located at the bolt head, from barrel extension, unlocking the bolt and allowing mechanism to cycle, extracting and ejecting empty cartridge case on its rearward motion and feeding fresh cartridge from magazine on its forward movement. Once the bolt comes into the battery (forwardmost position), the bolt carrier forces it to rotate and lock with the barrel extension. This action, popularly known as “Direct Gas Impingement” or DGI in short, has long been criticized as “dirty”, as powder gases are allowed to expand inside the gun mechanism. Over the time, this results in accumulation of powder fouling and carbon residue on moving parts, which, if not cleaned properly, will eventually result in malfunctions of the gun. This is especially true when using cheap, low quality ammunition and when critical parts (bolt, bolt carrier and barrel) are not chrome-lined or otherwise protectively coated. Properly built rifles with chrome-lined or stainless steel parts and fed with quality ammunition can shoot thousands of rounds before requiring routine maintenance (cleaning).

In attempts to improve on shortcomings of the Stoner-designed DGI system, several companies now offer piston-operated upgrades or complete upper receivers and rifles with gas piston actions. In these systems, gas tube is replaced by gas piston and rod, located above the barrel, inside the handguards. This way, powder cases are ejected into interior of the handguard after operating the piston, and no gases are allowed to enter interior of the bolt group. These systems are generally promoted as “cleaner, cooler and more reliable” than original DGI system, but for most civilian shooters there will be little to no practical difference in actual reliability, and minor difference in maintenance procedures, as DGI problems are most often encountered under extreme conditions of military combat. Another issue is that improperly designed gas piston conversions can actually enhance wear on the upper receiver, due to asymmetrical forces, applied by a gas piston rod to the bolt carrier. So, if you want a “Gas piston Ar-15”, you’d better to look for properly designed conversions, which address this and some other issues, inherent to the gas piston systems in Ar-15 platform.

Piston-operated AR-15-type carbine upper (top) compared to standard direct gas impingement carbine upper (bottom)

As mentioned above, the “body” of a typical Ar-15 rifle consists of two parts – upper receiver and lower receiver. Lower receiver (which, under US laws, is the only part that legally comprises a “firearm” and must be serially numbered) houses trigger unit, manual safety, and magazine housing. It also has interface for buffer tube (which contains bolt return spring and bolt buffer). Upper receiver houses bolt group and has interfaces to mount a barrel and rear sight. Per “Military specifications” both upper and lower receivers are forged from 7075-T6 aluminium alloy and had its surfaces hard anodized. Receiver halves are connected by two captive cross-pins, made of steel – one at the front and another at the rear. To disengage upper and lower receiver, one must push pins from left to right, then pull them all the way from the right, until upper receiver is free. Pins will remain in the lower receiver.

It must be noted that forged aluminium is not the sole material for AR-15 receivers. Several smaller companies in USA manufacture AR-15-type receivers by CNC machining solid blocks (billets) of aluminium, and few others (like Bushmaster or now defunct Cavalry Arms) offer lightweight lower and in some cases upper receivers made from polymer composites. In most cases, all these receivers are interchangeable if all critical dimensions are made to Mil-Spec.

short-barreled AR-15 type carbine assembled with lower receiver made from translucent plastic made by now-defunct Cavalry Arms company

just few of the AR-15 type upper receiver configurations offered by Vulcan arms

Barrels for Ar-15 type rifles can be made in wide variety of external profiles, lengths and calibres. Many manufacturers offer chrome-lined barrels, which have longer service life and better rust resistance (all current “Military issue” barrels are chrome lined as well). Other options may include stainless steel barrels or carbon steel barrels, which (in theory) can be made with more precision but require more extensive care and maintenance. “Mil-spec” barrels can be easily replaced with use of some special tools. Some manufacturers also offer “quick change barrel” conversions which usually require special barrel nuts and/or special receivers. Ar-15 barrels are often equipped with flash hiders or muzzle jump compensators, although recoil with standard .223 / 5.56mm ammo is minimal. Flash hiders become more and more useful as the barrel lengths become shorter. Some countries and states in USA allow for civilian ownership of sound moderators, commonly known as “sound suppressors” or “firearm silencers”. These devices can be used on most Ar-15 type rifles, and are especially useful for hearing protection when installed on short-barrelled carbines.

Typical trigger / safety setup of the Ar-15 style rifle is a single stage trigger which allows for single shots, and a manual safety lever with two positions (“Safe” and “Fire”), located on the left side of the lower receiver. Another standard feature of almost every AR-15 style rifle is the bolt catch (also known as the bolt hold-open device), which stops the bolt group in the open position once the last cartridge from magazine has been fired. This indicates that the gun is empty and facilitates faster reloading. There are two ways to disengage bolt catch once empty magazine is removed – shooter can press a bolt release button on the left side of receiver (faster method), or pull and release bolt handle (slower but allows for slow bolt closure if you wand to close it over empty chamber). Optional features may include ambidextrous safety levers, match-grade triggers, and ambidextrous magazine release buttons (which may or may not require modifications to the lower receiver).

Ar-15 rifles are fed from detachable box magazines. Magazine release button is located on the right side of the magazine housing, and some companies now offer ambidextrous magazine releases which require modifications to lower receiver. Mil-Spec (“US GI issue”, STANAG) magazines are normally made of aluminium and have capacity of 30 rounds (20 rounds for early, Vietnam War era M16 magazines). Commercially available magazines can have capacities from 5 to 100 or even 150 rounds, with 30 rounds being most common. Commercial magazines also can be made of aluminium, steel or plastic. It must be noted that bad (damaged or poorly built) magazines are one of most frequent sources of problems and stoppages, encountered with AR-15 style rifles. Therefore, it is advisable to buy best available magazines to ensure high reliability of your gun.

Standard furniture for Ar-15 rifles (shoulder stocks, pistol grips, handguards) is almost invariably made from some sort of plastic, and prevailing colour is black, which is responsible for the “Black rifle” moniker, attached to the whole M16 / Ar-15 family of weapons. Other colours, such as desert tan or various camouflage patterns are available as options from many manufacturers.

Shoulder stocks for Ar-15 can be had in many variations, but there are two basic “patterns” for these – fixed “M16-type” stocks and collapsible “M4 carbine type” stocks. Handguards also are available in variety of lengths (rifle or carbine) and designs. Many modern Ar-15 rifles have polymer handguards replaced by some sort of the rail interface, which typically is formed by an aluminium tube with several (one to four) Picatinny type accessory rails at 12-, 3-, 6- and 9-o’clock positions. These rails are used to mount additional accessories, such as optical sights, flash lights, laser aiming modules, forward grips, bipods and everything else. Typical railed handguards also are available in two major “flavours” – standard, which are attached to the upper receiver and barrel, and “free-floated”, which are attached to upper receiver only, allowing the barrel to “free float” inside the handguards. The latter style, while more expensive, generally allows for better accuracy and receives less heat from the barrel during prolonged shooting sessions.

Speaking on sighting equipment, original Ar-15 type sporting rifles closely followed military pattern, featuring protected front post sight mounted above the gas block, and an aperture-type rear sight, built into the integral carrying handle above the receiver. During mid-1990s, integral carrying handles gradually fell out of favour, first with military and then with everybody else. Instead, upper receivers of most currently made Ar-15 style rifles are shaped flat on top to form so called Picatinny (Mil-Std 1913) rail, which can be used to quickly and securely mount almost any type of sighting equipment. Evolution of the “Red Dot” (reflex, collimating) sights made them preferred sighting means for short- to medium ranges for most military and civilian applications. For more accurate work, magnifying optical sights can be easily installed on the same Picatinny rail using appropriate mounts. The bonus of using Picatinny rails is that several sights can be mounted on one rifle at the time, such as Red Dot plus back-up iron sights (BUIS) on folding bases, or Red Dot plus magnifier or Red Dot plus Night Vision monocular etc. For civilian applications this means that shooter can have several sights for one rifle, i.e. Red Dot for home defence and telescope sight for hunting, and change these quickly according to his current situation.