The ArmaLite (AR) 10 and 15 are both gas operated, rotating bolt semi-automatic rifles. The AR-10 was originally designed in the mid-1950s as a replacement rifle for the World War Two Era M-1 Garand rifle, but lost in competition to the Springfield Armory M-14. At the time, the preferred ammunition for U.S. military main battle rifles was the 7.62 mmx51/ .308 caliber cartridge, to ensure commonality with weapons of the North Atlantic Treaty Organization (NATO) alliance. The M-14 was basically an updated version of the M-1 Garand, including the tradition wooden frame and stock, but with fully automatic fire capability and a large detachable ammunition magazine. While the AR-10 did lose out to the M-14, it was revolutionary for its time, being extremely lightweight due to to its composite frame and stock and use of forged aluminum components. These design elements were used to develop the AR-15 model rifle, in 5.56 mmx45/.223 caliber in response to another U.S. Army proposal for a lighter rifle using the smaller ammunition, with the reasoning that a lighter weapon of a smaller caliber would be less of a burden for soldiers to carry, and would allow more ammunition to be carried as well. Unfortunately, factions within the Army delayed a timely decision on the 5.56 mm main battle rifle.
Transition to Colt
ArmaLite, having lost the competition to replace the M-1 Garand and encountering the Army’s indecision on the 5.56mm rife competition, sold the rights for both firearms to the Colt Manufacturing Company of Connecticut, an historic producer of weapons for the U.S. military. In 1961 the U.S. Air Force independently ordered several thousand AR-15 rifles from Colt for its Security Police forces, who are charged with the defense of nuclear munitions and nuclear armed aircraft. Eventually the Army adopted modified AR-15 models, redesignated as the M-16, as its main battle rifle in 1964.
Although the AR-15 was born as a variant of the AR-10, there are some significant differences (see: https://adventurefootstep.com/ar-10-vs-ar-15/) The main advantage of the AR-10 is the larger caliber ammunition, which means more “stopping” power, and better accuracy at longer distance. This makes the AR-10 a better option for hunting larger game. The main advantage of the AR-15 is the ergonomics of its lighter weight and smaller size, making it more suited to the urban battlefield. Since the adoption of the AR-15/M-16 variant as a U.S. main battle rifle, the increased availability of the AR-15 has generated a wide variety of additional features and components attractive to recreational shooters, and ammunition is less expensive. As the AR-10 was never adopted for large scale production, AR-10s are more expensive, and replacement parts and aftermarket components are limited. As with any comparison between firearms, the individual shooter and the main purpose the rifle will be used for should be the main factors in the decision of which rifle to purchase.