The AR-15 mag catch is the component that holds the magazine in the lower receiver of the rifle. It engages the magazine well and keeps the magazine locked in place during firing. Shooters press the mag release button to disengage the mag catch and drop the magazine. The mag catch works with standard AR-15 lower receivers and fits mil-spec magazine wells. Builders install it as part of the lower parts kit and pair it with the mag catch spring and mag release button.