The AR-15 magazine is a detachable box magazine that holds and feeds .223 Remington or 5.56 NATO cartridges into the rifle. Shooters load rounds into the magazine and insert it into the lower receiver mag well. The magazine catch locks it in place until the shooter presses the release button. AR-15 magazines come in various capacities and fit standard mil-spec lower receivers. Builders pair them with the bolt carrier group, mag catch, and lower parts kit to complete any AR-15 build.