The AR-10 is an assault rifle which fires 7.62mm NATO ammunition. Designed by Eugene Stoner at ArmaLite, the AR-10 was not widely used and built only to the tune of perhaps 10,000 examples. Nevertheless it is historically notable as being the basis for the later AR-15 design, which was adopted by the US Army as the M16. As such they became of coveted collectors item, and were recently put back into production for the civilian market.

ArmaLite first opened as a division of Fairchild in 1954, specifically to bring new materials and designs to the firearm industry. They quickly released a number of interesting designs. Later that year they were joined by Eugene Stoner, who started looking for new projects.

At the time the Army was in the midst of testing several rifles to replace the now outdated M1 Garand. Springfield Armory's T44 and heavier T45 were essentially cleaned up versions of the Garand chambered for the new 7.62mm round, while Fabrique Nationale submitted their FN FAL as the T48. Stoner decided that he could win the competition, even though they were late in testing at that point, and started work on what would become the AR-10.

The AR-10 combined features from a number of designs into one very modern design. From the FAL it took the hinged receiver area, which allowed the rifle to be "folded open" for cleaning. From the Ljungman AG42B it took the unique gas operating system. Most systems allowed the gas to move a short distance from a hole in the barrel into a chamber where it pushed on a rod that drove the bolt. In Stoner's design, the gas tube ran all the way to the receiver area, where it pushed a cam on the bolt itself. The result was less weight in the operating mechanism, and thus a higher rate of fire. The receiver itself was made from machined aluminum, dramatically reducing weight. The bolt locked into an extension on the barrel instead of the receiver in order to ensure a strong steel-on-steel fit. To further reduce weight, the barrel was made of a steel liner inside an aluminium barrel, a system used successfully on their earlier AR-7 survival rifle, and the flash hider was made of titanium.

The most noticeable difference in the AR-10 design was its layout. Traditional rifle design placed the barrel in line with the shooter's eye, with sights mounted on top. In order to transfer the recoil to the shoulder, the stock bent down behind the receiver, and was also used as the rear hand grip. However this leads to serious recoil problems, since the barrel is above the support point on the shoulder, every shot forces the barrel to rise.

For the AR-10 Stoner placed the barrel in-line with the stock, thereby reducing the climbing action. However this placed the shooter's eye well above the barrel, where the sights are normally mounted. He solved this problem by simply moving the sights upward, mounting them on a rail that also served as a convinient carrying handle. The charging lever was mounted under the handle, in the form of a upside-down trigger. The inline stock meant it was not usable as a grip as well, so he added a pistol-type grip instead. All of the "furnature" was made of a brown plastic, another innovation that reduced weight.

The AR-10 simply looked like nothing else in the world. When it was introduced into competition in 1955 it was technically clearly better than any of the other designs, and almost two pounds lighter than either. In testing the composite barrel, used successfully on their smaller caliber designs, proved to not be strong enough for the 7.62 rounds and two exploded. Stoner decided to replace it with a chromium-molybdenum-vanadium steel alloy instead, but the Army simply rejected the design instead. In retrospect it appears its "odd looks" had much more to do with the decision than the problems themselves. In the end the Army chose the conventional T44, which entered production as the M14 rifle in 1957.

A manufacturing license was sold to a Dutch company named Artillerie Inrichtingen. Production was limited, with only Sudan and Portugal purchasing AR-10 rifles for their militaries. Production of the AR-10 ceased during the early or mid-1960s and only around six thousand military AR-10's being made, and some additional number in semi-automatic mode for civilian use. When the M16 was adopted as the standard US rifle, the AR-10 gained cult status and became a covetted collectors item.

ArmaLite also started a program to produce "low cost" versions of both their AR-10 and AR-15 rifles, using stamped metal in the receiver area instead of milling. Known as the AR-16 and AR-18 respectively, the former was produced only in prototype quantities after the M16 was adopted, and the AR-18 saw only limited production.

In the early 1990s Eagle Arms purchased the ArmaLite brand name and re-introduced the AR-10 to the market. With the original brown plastic it is known as the AR-10A, and with M16A2-style furnature it becomes the AR-10A2 while the AR-10A4 has the "flat-top" style receiver with the Picatinny rail instead of the carrying handle, and the AT-10(T) is a match-grade version of the A4.