AR-15 lower parts springs are the small coil springs that work within the lower receiver to operate the fire control group and other components. They include the trigger spring, hammer spring, disconnector spring, safety detent spring, magazine catch spring, and bolt catch spring. Each spring provides tension to reset parts after each shot or to hold detents in place. Builders use these springs when completing a lower receiver or replacing worn parts in an existing build. They fit standard mil-spec AR-15 lower receivers.