AR-15 is the common name for the widely owned semi-automatic rifle that is derived from the selective fire M16-series assault rifle, currently in use by the United States military. ("AR-15" was the original name for the select-fire rifle before it was adopted as the M16. Today, however, "AR-15" is now used almost exclusively to refer to semi-automatic versions of the M16-series rifles.) Shown here is a "Carbine" version which signifies a 16" barrel and a collapsable stock, The "Rifle" version typically has a 20" or longer barrel and a fixed length stock.

Standard AR-15 rifles accept detachable magazines of widely varying capacities,however the California compliant models have a fixed magazine with a 10 round maximum capacity 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, folding or 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, etc.

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. These can be purchased seperately to but the lower receiver is a controlled part and considered a firearm by itself while the  upper receiver assembly is simply considered a part, and may be freely purchased with no registration needed. This is a very attractive feature for enthusiasts, who often purchase a number of upper receivers (often in different calibers) and interchange them 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. Sierra Gun Works can help you with the legalities of a custom build and assist with any parts you may need.