Some notable features of the AR-15:

Quality 7075-T6 aircraft-grade aluminum, lightweight, highly corrosion-resistant, and machinable;

Modular design, which allows the use of numerous accessories (sights, vertical grips, lighting systems, night vision systems, laser targeting devices, muzzle brakes, silencers, bipods, etc.) and facilitates repair;

Small caliber, lightweight, high-velocity bullet (.223 Remington/5.56x45mm);

Easily adaptable to fire other ammunition;

Adjustable front sight;

Adjustable rear sight for windage (on most models) and elevation (on some models);

Wide range of optical sights available to complement or replace the standard sight;

Short- or long-stroke gas piston borrow system, or direct bolt recoil systems available; 

Synthetic grip and stock that won't swell under adverse conditions (regulated in some states);

Various magazine capacities, ranging from 10 to 30 rounds or more;

Ergonomic design that makes the charging handle, selector switch (which also engages the safety), and magazine release button easy to access;

4 MOA accuracy (MILSPEC standard) (MOA = Minute of Arc, a unit of angle measurement equivalent to one-sixtieth of a degree).

AR-15 semi-automatic rifles intended for sale to civilians have internal differences from the fully automatic M16s, to which they are almost identical in appearance. The hammer and trigger mechanisms are of a different design. The bolt carrier and lower inner receiver of the semi-automatic versions are milled differently, so the firing mechanisms are not interchangeable.  This was implemented to meet the requirements of the U.S. Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF), which stipulates that civilian firearms cannot be easily converted to automatic.

Automatic versions have a three-position rotary fire selector that allows the operator to choose between three modes: safe, semi-automatic, and full-auto or three-round burst, depending on the model. Civilian versions have only two positions: safe and fire.