From AR-15 to XM16

Caliber: 5.56×45mm M193

When the "Project SALVO" was summarized in 1957, a study similar to the one abandoned by Gustafson in 1955 due to lack of funding was revived. General Willard G. Wyman, the commander of the U.S. Continental Army Command (CONARC), immediately requested manufacturers to organize research teams to develop a .22 caliber (5.6mm) rifle. The design requirements were as follows, but the last requirement indicates that the U.S. Army did not fully understand ORO's research results:

- 20-round capacity magazine

- Weight less than 6 pounds (2.7 kg) when loaded

- Selective fire capability

- Ability to penetrate a standard U.S. steel helmet, body armor, or 0.135-inch (3.4mm) steel plate at a range of 500 yards (about 460m), with significantly greater "stopping power" than the .30 carbine rifle.

Upon seeing the AR-10, General Wyman personally suggested that Armalite should attempt to design such a weapon. Based on these requirements, Armalite's design team led by Eugene Stoner (chief designer), Robert Fremont (supervising prototype production), and L. James Sullivan (supervising the drafting of the scheme) began developing a new .22 caliber rifle. Their first design, however, which used a traditional layout and wooden stock (perhaps due to concerns arising from the failures of the AR-10 in trials), was found to be too light during testing. Moreover, the traditional stock design made it difficult to control recoil during rapid fire, even with lighter ammunition. So their second design decided to scale down the AR-10 to a caliber of .222 for an automatic rifle, and the trial was immediately successful. The new rifle was named the AR-15, and the design team submitted six prototype rifles to the Aberdeen Proving Ground for evaluation in just six months.

Meanwhile, Winchester Repeating Arms also joined the .22 caliber rifle competition with their Light Weight Military Rifle (LMR), which was based on their M1 carbine design. Eli Harvey of the Springfield Armory also developed another .224 caliber rifle prototype based on the T44 rifle, but Springfield's leadership refused to allocate resources from the T44 program to participate in this "uncertain" competition, so Springfield did not invest resources in the small caliber rifle.

The prototype gun developed by Winchester was a .224 caliber light military rifle (LMR), which was improved on the basis of the M1 carbine.

Springfield Armory's .224 caliber prototype based on the M14 rifle

.222 caliber AR-15 prototype

From left to right: 5.56×45mm predecessor Remington .222 SPL rifle bullet; Winchester .224E1 (5.56×43mm) bullet, Springfield Armory .224 (5.6×47mm) bullet

The earliest AR-15 prototype

Two early AR-15 prototypes

A series of AR-15 prototypes and early M16s collected by the Colt Museum

In the trials, to meet the last requirement of 500-meter penetration capability, Stoner requested Remington to improve their .222 Remington rifle cartridge, which had an initial velocity of 3300 feet per second (1006 m/s) and a 55-grain weight. The newly developed ammunition was the famous .223 Remington rifle cartridge. In addition to the caliber change, the barrel of the AR-15 was changed to a fully steel simple structure barrel, and a short cylindrical flash suppressor was adopted. The hook-shaped charging handle originally located inside the carrying handle was changed to a T-shaped charging handle behind the carrying handle. Other than the caliber, the AR-15's design incorporated features from various other rifles, including:

- The locking mechanism of the M1941 Johnson rifle

- The receiver cover and buttstock of the FN FAL

- The coaxial buttstock of the T-25

- The carrying handle with sight of the British EM2

- The dust cover for the ejection port of the German MP44 assault rifle

- The gas system of the Swedish Ljungman rifle, although Stoner's design simplified the gas system by reducing several moving parts, making rifle operation simpler and production costs lower.

Additionally, due to the lighter new ammunition allowing the rifle to use lightweight materials such as aluminum alloy and plastic, the size was reduced. Even with a full steel barrel, the AR-15 was lighter than the Winchester LMR by about 1 pound (about 450g), with the AR-15 weighing only 6.35 pounds (2.89 kilograms) when empty and about 3.5 kilograms when fully loaded. In the end, Armalite's AR-15 had no real competitors. However, Stoner also made some mistakes. He believed that it was not necessary to chrome plate the barrel, and at that time, Armalite did not have the technology to chrome plate the barrel. But research by the military during World War II showed that chrome-plating the barrel was very beneficial for automatic weapons. Therefore, the military announced that starting from the M14 rifle, all rifles produced should be chrome-plated.

Another issue was the short development time. Perhaps under pressure from the success of the M14 rifle, proponents of the small caliber rifle program hoped to reap the rewards quickly. So when the Army began testing the AR-15 nine months after its launch on March 31, 1958, the complete development process of the AR-15 had not yet been completed, and there had been no face-to-face work between Army ordnance engineers and Armalite designers throughout the process.

From 1958 to 1959, the Army conducted comparative trials of the AR-15 and the M14 at various locations such as Benning, Ord, and Aberdeen Proving Grounds. Through these trials, it was shown that the AR-15 was lighter, easier to handle, and easier to disassemble and assemble compared to the M14. Infantry departments reported that the AR-15 would be a suitable replacement for the M14 rifle. However, later tests revealed some problems, such as large muzzle flash, poor accuracy at long distances, poor penetration, and high failure rates when using 30-round magazines. Additionally, rainwater entering the AR-15 barrel caused bursting during firing. The rainwater-induced bursting problem also occurred with Winchester's prototype rifles, so although Stoner redesigned and reinforced the barrel, Army ordnance engineers said the problem still existed. They said that this was an expected problem for .22 caliber barrels, so the Army once again requested larger ammunition, this time suggesting .258 (6.55mm), but they hinted that reliability tests in polar environments would continue in Alaska. Then, Stoner heard that there were major problems with the AR-15 in the polar testing, but when Stoner flew to Alaska with some replacement parts, he found that the problem lay with the Army test personnel, who did not reassemble the rifle correctly after disassembling it. Stoner believed this was due to improper operation and requested retesting. After that, the military began to hinder Stoner's improvements to the AR-15, but said that if the AR-15 were modified to fire the new .258 caliber, which the ordnance department said was the best ammunition, it might be more suitable. In short, the U.S. Army lost interest in the AR-15, and the AR-15 still needed further development work. After reviewing these reports, General Maxwell Taylor, Chairman of the Joint Chiefs of Staff, also did not support the AR-15 and recommended continued production of the M14.

But not all reports were negative. In a series of simulated combat scenarios involving the AR-15, M14, and AK-47, the military found that the smaller size and lighter weight of the AR-15 made it more flexible and responsive. As CONARC had already proposed, their conclusion was that the firepower of an 8-person squad equipped with the AR-15 was equivalent to that of a 9-person squad equipped with the M14. Even General Wyman, who had supported the M14 program since the beginning of the competition, admitted that the AR-15 had advantages in the jungle environment of Vietnam. However, General Wyman still doubted that the AR-15 could penetrate the body armor of the Soviet soldiers, which was made of plates of heavy steel, so he did not allow further development of the AR-15.

Although the M16 has the nickname "Black Rifle", the handguard and buttstock of Colt's earliest production model 601 are actually dark green.

The 601 model is "flat" around the magazine release button, without a "protrusion" to prevent accidental operation, and the 602 model is the same

The 601 Colt AR-15, the early magazine in the picture was still Armalite steel magazine, and later it was changed to aluminum alloy magazine. Because of the shape of the grooves on the surface, this magazine was nicknamed "Waffle"

Model 601 three-pronged flash suppressor, commonly known as the "Duckbill"

The three-pronged flash suppressor of the 602 model has a frustum-shaped outer contour, which is different from the 601 model.

The bolt handle of the 601 model is triangular in shape, not the later T-shaped one.

The inscription on the 601-type AR-15 receiver that was just handed over from Armalite to Colt for production. It has both the Colt and Armalite logos on it.

Although the official model in the factory is called 601, the receiver is printed with "MODEL 01", so it is often referred to as 01.

To prevent the handguard from overheating and becoming difficult to hold after continuous shooting, a white aluminum alloy heat shield is fixed to the inside of the handguard with screws.

Therefore, even if the Army eventually abandoned the AR-15 program, General Curtis LeMay of the U.S. Air Force decided to arm his air police, sentries, and missile security personnel with the AR-15 to protect them from raids. He recognized the AR-15 as the best rifle for his troops. In 1959, Colt's Manufacturing Company of Hartford, Connecticut purchased the rights to produce AR-15 rifles from Armalite. At that time, Colt was already producing AR-15 rifles for the U.S. Air Force. In 1960, the U.S. Air Force ordered 8,500 AR-15 rifles, and in 1962, the U.S. Air Force ordered 19,000 AR-15 rifles. At the same time, the Army ordered Colt to produce a very similar rifle to the AR-15, the XM16E1, with some changes. The designation "XM16E1" indicates that it is a model of rifle submitted for evaluation ("X" stands for "experimental"), it is the first model of the M16 family, and it is the first model of the XM16 family. The U.S. Air Force's AR-15 rifle was modified in the following ways and officially adopted as the M16 rifle in 1964:

- Chrome plating of the barrel to resist corrosion from rain and snow

- Modification of the firing system, bolt carrier, and bolt locking system

- The AR-15's automatic and semi-automatic firing modes were changed to semi-automatic and automatic firing modes.

- The addition of a forward assist device on the receiver to assist with the forward movement of the bolt carrier when the cartridge is chambered

- The magazine well was modified to prevent the magazine from being accidentally released

- Modifications were made to the magazine to reduce jams

- The rate of fire was changed from 800 rounds per minute to 750 rounds per minute.

In 1965, U.S. President Lyndon B. Johnson announced that the AR-15 and M14 rifles would be combined into one weapon and the AR-15 rifle would be adopted by the U.S. military as the "M16 rifle." The U.S. military initially ordered 85,000 M16 rifles, but in 1966, the military adopted the M16A1 rifle, which was improved from the M16 rifle. In 1967, the U.S. Army replaced the M14 rifle with the M16A1 rifle, and in 1969, the U.S. Marine Corps also replaced the M14 rifle with the M16A1 rifle. From 1968 to 1970, the Army conducted an M16A1 rifle upgrade program to repair the problems of the M16 rifle and M16A1 rifle, and the M16A1 rifle has since been continuously improved.

Model 601 AR-15 used by Navy SEALs in Vietnam