AR-15 variants are based on the 762.mm designed by Eugene Stoner, Robert Fremont and James Sullivan of the Fairchild Armalite corporation. It was developed as a lighter mm version of the AR-10. The “AR” in all ArmaLite pattern firearms simply stands for “ArmaLite Rifle” and can be found on most of the company’s firearms: AR-5, a .22 caliber rifle; the AR-7, another .22 caliber; the AR-17 shotgun; the AR-10 rifle; and the AR-24 pistol.
In 1959, ArmaLite sold its rights to the AR-10 and AR-15 to Colt. After a tour by Colt of the Far East, the first sale of AR-15s was made to Malaya on September 30, 1959. Colt manufactured their first 300 AR-15s in December 1959. Colt marketed the AR-15 rifle to various military services around the world. After modifications (most notably the relocation of the charging handle from under the carrying handle to the rear of the receiver), the redesigned rifle was adopted by the United States military as the M16 rifle.
1973 Colt’s SP1 rifle with ‘slab side’ lower receiver (lacking raised boss around magazine release button) and original Colt 20-round magazine. This became the civilian version we know today.
Colt started selling the semi-automatic version of the M16 rifle as the Colt AR-15 for civilian use in 1964 and the term has been used to refer to semiautomatic-only versions of the rifle since then. Colt continued to use the AR-15 trademark for its semi-automatic variants (AR-15, AR-15A2) that were marketed to civilian and law-enforcement customers. The original AR-15 was a very lightweight weapon, weighing less than 6 pounds with empty magazine. Later heavy-barrel versions of the civilian AR-15 can weigh upwards of 8.5 lb.
Initial sales of the AR-15 were slow, primarily due to its fixed sights and carry handle that made mounting a scope difficult and awkward to use. This changed in the 1990s with the introduction of the flat top upper receiver and new features such as free floating hand guards that increased accuracy.
Only a handful of companies were manufacturing these types of rifles in 1990’s, but by 2010 over 100 companies are making some variant of the AR. This rifle is now known as the modern sporting rifle and is here to stay.