The AR-15 mag catch spring is a small coil spring that provides tension to the magazine catch in the lower receiver. It keeps the magazine catch engaged so the magazine stays locked during firing. Shooters press the mag release button against this spring to drop the magazine. The mag catch spring installs as part of the lower parts kit and fits all mil-spec AR-15 lower receivers. Builders replace it to restore reliable magazine retention in any AR-15 build.
