It's been called "America's Gun." The semi-automatic weapon known as the AR-15 - sometimes referred to by its many fans as "the Black Rifle"- is by some estimates the most popular rifle in the United States, with an estimated 4 million in circulation. And it's back in the news once again.
The AR-15 is a lightweight, 5.56 mm/.223-caliber, magazine-fed, air cooled rifle with a rotating-lock bolt, actuated by direct impingement gas operation or long/short stroke piston operation. It has been produced in many different versions, including numerous semi-automatic and selective fire variants. It is manufactured with extensive use of aluminum alloys and synthetic materials.

The AR-15 was first built by ArmaLite as a small arms rifle for the United States armed forces. Because of financial problems, ArmaLite sold the AR-15 design to Colt. After modifications (most notably the relocation of the charging handle from under the carrying handle like the AR-10 to the rear of the receiver), the new redesigned rifle was subsequently adopted as the M16 rifle. Colt then started selling the semi-automatic version of the M16 rifle as the Colt AR-15 for civilian sales in 1963 and the term has been used to refer to semiautomatic-only versions of the rifle since then. Although the name "AR-15" remains a Colt registered trademark, variants of the firearm are independently made, modified and sold under various names by multiple manufacturers.