The AR-15 (ArmaLite Model 15) is a widely owned semi-automatic rifle, of which the most famous derivative is the selective fire M16-series assault rifle used by the United States military.
The AR-15 consists of separate upper and lower receiver assemblies, which are attached with two through-pins and can be quickly interchanged with no tools. The lower receiver (because it bears the weapon's serial number and fire control group) is itself regulated as a firearm. However, the upper receiver assembly is simply considered a part, and may be purchased and mail-ordered in most locations with no restrictions. This is an attractive feature for users because it allows a number of upper receivers (often in different calibers) to be interchanged with the same lower receiver. However, one must be thoroughly familiar with firearms laws before doing this as it is possible to make an illegal configuration.
Standard AR-15 rifles accept detachable magazines of widely varying capacities, and have a pistol grip that protrudes beneath the stock. AR-15 rifles are highly configurable and customizable. They are commonly fitted with several accessories such as bipods, collapsing stocks, threaded barrels for the attachment of a flash suppressor, and a rail system for the attachment of vertical grips, flashlights, laser sights, telescopic sights and other accessories.