original military issue AR-15 / M16 rifle (circa 1965), with no forward assist and with the original three-prong flash hider

M16A1 rifle, with the forward assist, "bird cage" flash hider and the 20 rounds magazine (1967)

M16A1 rifle with 30 rounds magazine and a 40mm M203 grenade launcher attached (circa mid-1970s)

M16A2 rifle, with the heavy barrel, modified rear sights, spent case deflector, round handguards and modified pistol handle (circa mid-1980s)

M16A3 "flat top" upper receiver with the Picatinny rail and the removable carrying handle

Speak about the flexibility: just three of many dozens upper receiver options available on the market for the commercial and military AR-15 type rifles.

	M16A1	M16A2
Caliber	5.56x45mm (.223 Remington), M193	5.56x45mm NATO / M855
Action	gas operated, rotating bolt
Overall length	986 mm	1006 mm
Barrel length	508 mm	508 mm
Weight, empty / loaded w. 30 rounds	2.89 kg / 3.6 kg	3.77 kg / 4.47 kg
Magazine capacity	20 or 30 rounds standard
Rate of fire, cyclic	650 - 750 rounds per minute	800 rounds per minute
Muzzle velocity	945 m/s	975 m/s
Maximum effective range	460 meters	550 meters

The history of the development, introduction and the service of the US Rifle, 5.56mm, M16, is a long and a controversial one. I'll try to cut this story as short as possible, and will highlight only some most important periods and events. So, let's start.

• 1948. U.S. Army's Operations Research Office (ORO) conducts a research about small arms effectiveness. This research was completed by the early 1950 with the conclusion that the most desirable infantry small arms should be of 22 caliber, select-fire and with high velocity bullets, effective up to 300 meters or so.
• 1953 - 1957. US DOD conducts the next research, "Project SALVO", that also lead to the desirability of .22 caliber high-velocity infantry rifle
• 1957. The US Army requests the Armalite Division of the Fairchild Aircraft Corp to develop a rifle of .22 caliber, lightweight, select-fire, and capable to penetrate the standard steel helmet at 500 meters. The Eugene Stoner, then a designer at the Armalite, began to develop this rifle, based on his earlier design, 7.62mm AR-10 battle rifle. At the same time, experts at the Sierra Bullets and the Remington, in conjunction with Armalite, began do develop a new .22 caliber cartridge, based on the .222 Remington and .222 Remington Magnum hunting cartridges. This development, initially called the .222 Remington Special, was finally released as .223 Remington (metric designation 5.56x45mm).
• 1958. Armalite delivers first new rifles, called the AR-15, to the Army for testing. Initial tests display some reliability and accuracy problems with the rifle.
• 1959. Late that year Fairchild Co, being disappointed with the development of the AR-15, sold all rights for this design to the Colt's Patent Firearms Manufacturing Company.
• 1960. Eugene Stoner leaves the Armalite and joins the Colt. The same year Colt demonstrated the AR-15 to the US Air Force Vice Chief of Staff, Gen. LeMay. Gen. LeMay wanted to procure some 8 000 AR-15 rifles for US AF Strategic Air Command security forces to replace ageing M1 and M2 carbines.
• 1962. US DoD Advanced Research Projects Agency (ARPA) purchases 1000 AR-15 rifles from Colt and sends those rifles to the South Vietnam, for field trials. Same year brings glowing reports about the effectiveness of the new "black rifle", used by South Vietnamese forces.
• 1963. Colt receives contracts for 85 000 rifles for US Army (designated as XM16E1) and for further 19 000 rifles for US Air Forces (M16). The US AF M16 was no more than an AR-15 rifle with appropriate markings. The XM16E1 differed from AR-15/M16 by having an additional device, the so called "forward assist", which was used to manually push the bolt group in place in the case of jams.
• 1964. US Air Forces officially adopted new rifle as M16. Same year US Army adopted the XM16E1 as a limited standard rifle, to fill the niche between discontinued 7.62mm M14 rifle and the forthcoming SPIW system (which newer got past the prototype and trial stages).
• 1966. Colt was awarded with the contract for some 840 000 rifles for US Armed forces, worth almost $92 millions.
• 1967. US Army adopted the XM16E1 rifle as a standard "US Rifle, 5.56mm, M16A1", on 28 February 1967.
• 1965 - 1967. Field reports from Vietnam began to look much more pessimistic. M16 rifles, issued to US troops in the Vietnam, severely jammed in combat, resulting in numerous casualties. There were some causes for malfunction. First of all, during the introduction of the new rifle and its ammunition into the service, US Army replaced originally specified Dupont IMR powder with standard ball powder, used in 7.62x51mm NATO ammunition. The ball powder produced much more fouling, that quickly jammed the actions of the M16 unless the gun was cleared well and often. This pitifully combined with the fact that the initial M16 rifles were promoted by the Colt as "low maintenance", so, for the sake of economy, no cleaning supplies were procured for new M16 rifles, and no weapon care training was conducted fro the troops. As a result, soldiers did not knew how to clean their rifles, and had no provisions for cleaning, and thing soon turned bad. To add the trouble, the ball powders also had a different pressure curve, so they produced higher pressures at the gas port, giving the rise to the rate of fire, and, thus, decreasing accuracy and increasing parts wear.
• 1967 - 1970. The deficiencies discovered in previous years began do dissolve. 5.56mm ammunition was now loaded using different powders that produce much less residue in the gun action. The barrel, chamber and bolt of the rifles were chrome-lined to improve corrosion resistance. Cleaning kits were procured and issued to troops, and a special training programs were developed and conducted ever since. Earliest cleaning kits could be carried separate from rifle only, but since circa 1970 all M16A1 rifles were manufactured with the containment cavity in the buttstock, that held the cleaning kit. At the same time (circa 1970) the new 30 rounds magazines were introduced into service instead of the original 20 rounds ones, to equal Soviet and Chinese AK-47 assault rifles, which had 30-rounds magazines from the very beginning.
• 1977 - 1979. NATO trials lead to the adoption of the improved 5.56x45mm cartridge, developed in Belgium by FN. This cartridge, initially developed in conjunction with the FN Minimi light machine gun, featured a slightly heavier bullet with accordingly slightly lower muzzle velocity. The resulting long-range performance, however, improved due to the better ballistic coefficient of the new bullet. The SS109 required a faster rifling twist to stabilize its bullet, than the original 5.56x45mm US M193 ammunition. The M193 was used with barrels rifled with 1:12 twist (1 turn in 12 inches), and SS109 was preferred to be fired with 1:7 twist (1 turn in 7 inches). Some arms manufacturers preferred to make their guns with intermediate 1:9 rifling, which would be equally good (or bad) for both old and new loadings.
• 1981. Colt developed a variation of the M16A1, adapted for the SS109/5.56mm NATO cartridge, and submitted it to the military trials as the M16A1E1. This rifle differed from the M16A1 by having the heavier barrel with faster 1:7 rifling, a different type rear sights (adjustable for both range and windage), round handguards instead of triangular ones, and by replacing the full-auto fire mode with the burst (limited to 3 rounds per trigger pull), to preserve the ammunition.
• 1982. M16A1E1 is type-classified by US DoD as the "US Rifle, 5.56mm, M16A2".
• 1983. US Marine Corps adopted the M61A2 rifle.
• 1985. US Army officially adopted the M16A2 as the general issue infantry rifle.
• 1988. The FN Manufacturing Co, an US subsidiary of the FN Herstal (Belgium), becomes the key contractor to US DoD for production of the M16A2 rifles. Colt continues the development and manufacture of the AR-15 / M16 type rifles only for civilian and law enforcement markets from that point.
• 1994. Adoption of the latest variations of the M16 breed. Those include: M16A3and M16A4 rifles, with "flat top" receivers, that had a Picatinny accessory rails in the place of the integral carrying handle. The rail can be used to mount detachable carrying handle with iron rear sights, or various sighting devices (Night/IR, optics etc). The M16A4 otherwise is similar to M16A2, while M16A3 has a full-auto capability instead of the 3-rounds burst. Two other newest AR-15 offsprings are the M4 and M4A1 carbines, which are described in the separate article on this site.

The M16 is still a general-issue rifle with the US Armed forces. It is also widely used by the US Law Enforcement agencies, either in military form (for example, the LAPD had some M16s, retired from Army), or in "civilian" semi-automatic only form. The AR-15 style rifles are made in the USA by at least dozen large companies, such as Armalite, Bushmaster, Colt, FN Manufacturing, Hesse, Les Baer, Olympic, Wilson Combat, and by number of smaller companies, many of which do assembly their rifles from components made by some other major manufacturers. M16-type rifles also manufactured outside of the USA, most notably in the Canada, by Diemaco Co. China also makes some AR-15 type rifles at NORINCO state factories. M16 rifles are used by many foreign military groups, most notably the British SAS, who preferred the M16 over the infamous L85A1 rifle, and by many others.
At the present time almost all initial flaws of the M16 are bugged out, and it is considered among the best assault rifles in the world. While its reliability in the harsh conditions cannot match reliability of its main rival, the Kalashnikov AK-47 and AK-74, it is still a quite reliable rifle, especially when well maintained. It is also comfortable to fire and quite accurate.
One of the key advantages of the Stoner design, that must be especially stressed, is the extreme flexibility of the construction. At the present time the interchangeable complete "uppers" are available in various barrel lengths and profiles (from 7 to 24 inches long, slim and heavy), in dozens of rifle and pistol calibers (from tiny but fast .17 Remington and up to monstrous .458 SOCOM, and from .22LR and 9mm Luger up to mighty .50AE). Special, manually single-shot uppers are commercially available in the extremely powerful .50BMG (12.7x99mm) caliber. Various "lowers" offer a broad variety of trigger units, buttstocks and other options. This advantage is viable for both military (especially Spec Ops), Law Enforcement, and civilian applications, as it allows to tailor any particular AR-15 type rifle to the current situation and tactical needs.
M16 / AR-15 Technical description
The original AR-15 rifle is a gas operated, selective fire, magazine fed weapon. Every rifle from the M16 family is generally the same, but most civilian AR-15 type rifles are semi-automatic only.
The heart of the AR-15 is the direct gas system, developed by the Eugene Stoner in the early 1950s. This system uses no conventional gas piston and rod to propel bolt group back after the shot is fired. Instead, the hot powder gases are fed from the barrel and down to the stainless steel tube into the receiver. Inside the receiver, the rear end of the gas tube enters into the "gas key", a small attachment on the top of the bolt carrier. The hot gases, through the gas key, enter the hollow cavity inside the bolt carrier, and expands there, acting against the bolt carrier and the collar around the bolt body. The pressure of the gases causes the bolt carrier to move back against initially stationary bolt. The linear rearward movement of the carrier initially transferred into the rotation of the bolt, via the cam slot in the bolt carrier and the cam pin, attached to the bolt, that followed the slot. As soon as the bolt is rotated to unlock from the barrel, the bolt group continues its rearward travel under the inertia and the residual pressure in the barrel, extracting the spent case and compressing the buffer return spring, located in the buttstock. The forward movement of the bolt group first strips the fresh cartridge from the magazine and, on the final stage of the movement, rotates the bolt to lock into the barrel extension. The bolt has 7 radial locking lugs, eight lug is located on the extractor claw. Since the introduction of the XM16E1 rifle, the forward assist device is used on all military and most civilian AR-15 type rifles. This device consist of the spring-loaded button with internal claw, that engages the serrations on the right side of the bolt carrier to push it forward, if the pressure of the return spring is insufficient to do so (for example, due to the fouling inside the receiver or chamber). The rifle will not fire unless the bolt is locked and the bolt carrier is in its forwardmost position. The bolt carrier and the bolt itself are chrome-plated. Another feature of the AR-15 type rifles is the bolt catch device, that locks the bolt group in the open position when the last round is fired. To release the bolt group one must push the button, located at the left side of the receiver, above the magazine. The "T"-shaped cocking handle is located at the rear of the receiver, above the buttstock, and does not reciprocate when gun is fired.
The trigger/hammer group is basically similar to one, found in M1 Garand rifle, and, actually, traces its roots back to the early 1900s, when the great John M. Browning developed his famous Auto-5 semiautomatic shotgun. This basically consists of a hammer, a trigger, a disconnector, a full auto sear and some springs. The fire selector / safety switch is located at the left side of the receiver, above the pistol grip, and is easily operated by the right hand thumb. This switch has 3 positions: "safe", "semi" (single shots), and "auto" (full automatic on M16A1 and A3) or "burst" (3 rounds bursts, on M16A2 and A4). In the latter case (on the M16A2 and A4 rifles), the trigger unit also includes the ratchet device to count the shots fired.
The ejection port is located at the right side of the receiver, and is closed by the spring-loaded dust cover, which automatically pops open when bolt carrier is pulled back. The M16A2 also featured the spent case deflector - a triangular bulb on the receiver, just behind the ejection port, that allows the gun to be safely fired left-handed.
The M16 is fed using box magazines. Earliest magazines were made from aluminum and held 20 rounds. Circa 1970 the new, 30 rounds magazines were introduced into service and these magazines are still in service now. An extremely wide variety of magazines available on the commercial marked, starting from the "US post-ban" 5 and 10 round magazines, and up to 40-rounds box, 90-rounds helical, 100-rounds dual drums (Beta-C) and 120-rounds single drums.
The receiver is made from aluminum alloy, and consists of two parts - "upper receiver" and "lower receiver" (sometimes referred simply as "upper" and "lower"). Most receivers are made from aluminum forgings by machining, but some commercially available receivers are made from aluminum castings with final drilling and machining. The upper and lower receivers are linked by two cross-pins - one at the front (pivot pin), and one at the rear, above the pistol grip (takedown pin). To field strip the AR-15, one must push the rear pin to the right as far as it will go, and then hinge the upper receiver around the front pin. This will allow the bolt group and the carrying handle to be removed from the upper receiver. For further disassembly, the front pin also must be pushed out, and the upper and lover receiver can be separated. The key benefit of this design is the great flexibility - if all components available are made to the same specifications (in most cases they are), one can easily swap various upper receivers on one lower receiver and vice versa. Since the complete "upper" module consist also of the bolt group and the barrel with the gas system, one can easily have different barrel lengths, styles (light, heavy, fluted, bull), and even calibers, for one "lower" group, that consists of the lower receiver with the trigger/hammer unit, recoil buffer, pistol grip and the buttstock.
The furniture on military rifles is made from the black plastic, hence the common name "the black rifle". On the early AR-15 and M16A1 rifles, the handguards were of triangular cross-section, and were made from two non-interchangeable parts. On the M16A2 and latter rifles, the handguards are of round cross-section, and have two interchangeable upper-lower sections. The buttstock on the M16A2 is similar in design to one of M16A1, but slightly longer. The one disadvantage of the Stoner system is that it can not be adapted for conventional folding buttstock. Instead, if required, a telescoped stock is used, that allows to shorten the rifle when required by about the half of the length of the standard stock. M16 is usually equipped with sling, and can accept a knife - bayonet, either an old style M7, or a newer style M9. The flash hiders on the earliest AR-15s and M16s were prong-type, with three open slots, but later were replaced with "bird-cage" flash hiders with four (M16A1) or five (M16A2) slots.
Both M16A1 and M16A2 can be equipped with underbarrel 40mm M203 grenade launcher. M203 mount replaces the standard handguards on the rifle and requires a grenade launcher sight to be mounted on the carrying handle.
Standard sights of the M16A1 consist of a protected front post, mounted on the gas block, and of an aperture flip-up rear, with 2 range settings. Rear sights are mounted within the carrying handle and are adjustable for windage. The A2 style rear sight also features an flip-up, dual aperture sights, with one smaller aperture for daylight usage, and another larger aperture for low light conditions. The range adjustments are made by the rotating knob, located just under the sight. The front sight is generally the same as on the M16A1. The M16A3 and A4 rifles have detachable carrying handles with A2 sights, and the Picatinny-type MilStd rail on the top of the receiver, that can accept a wide variety of sighting devices and mounts.