AR-15 lower parts are the individual components that make up the lower receiver assembly of the rifle. They include the trigger, hammer, disconnector, springs, pins, safety selector, magazine catch, bolt catch, and buffer components. Builders use these parts to complete a stripped lower receiver or replace worn components in an existing build. Each part fits standard mil-spec lower receivers and works with AR-15 upper receivers and magazines. Shooters select parts individually or purchase a kit to cover all the lower har-15 lower partsardware at once.