In 1956, ArmaLite designed a lightweight selective fire rifle for military use and designated it the ArmaLite Rifle model 15, or AR-15.
Comparison to military versions
The semi-automatic civilian AR-15 was introduced by Colt in 1963. The primary distinction between civilian semi-automatic rifles and military assault rifles is select fire. Military models were produced with firing modes, semi-automatic fire and either fully automatic fire mode or burst fire mode, in which the rifle fires several rounds in succession when the trigger is depressed.
Most components are interchangeable between semi-auto and select fire rifles including magazines, sights, upper receiver, barrels and accessories. The military M4 carbine typically uses a 14.5-inch (37 cm) barrel. Civilian rifles commonly have 16-inch (41 cm) or longer barrels to comply with the National Firearms Act.
In order to prevent a civilian semi-automatic AR-15 from being readily converted for use with the select fire components, a number of features were changed. Parts changed include the lower receiver, bolt carrier, hammer, trigger, disconnector, and safety/mode selector. The semi-automatic bolt carrier has a longer lightening slot to prevent the bolt’s engagement with an automatic sear. Due to a decrease in mass the buffer spring is heavier. On the select fire version, the hammer has an extra spur which interacts with the additional auto-sear that holds it back until the bolt carrier group is fully in battery, when automatic fire is selected. Using a portion of the select fire parts in a semi-automatic rifle will not enable a select fire option (this requires a registered part with the ATF).
Lower receivers that are select fire are easily identified by a pin hole above the safety/mode selection switch. As designed by Colt the pins supporting the semi-auto trigger and hammer in the lower receiver are larger than those used in the military rifle to prevent interchangeability between semi-automatic and select fire components.