The AR-15 magazine catch button is the button on the left side of the lower receiver that releases the magazine. Shooters press the magazine catch button with their trigger finger to drop an empty magazine during a reload. The button threads into the magazine catch body and works with the magazine catch spring to lock and release AR-15 magazines. It installs as part of the lower parts kit and fits all mil-spec AR-15 lower receivers. Builders replace it to customize or repair the magazine release.