The ARMS #40 Flip-Up Sight is a backup rear sight that uses standard A2 apertures. The unique design of the ARMS #40 flip up spring loaded rear sight ensures constant in battery action and repeat zero. The ARMS #40 sight is held in the locked down position by a quick release lever, that prevents the sight from being accidentally engaged, but is very fast to release even with arctic mittens. The ARMS #40 has a 250-300 meter aperture for immediate use. For longer ranges the aperture is rotated to access the 500-600 meter capability. They also have the standard military horizon line notch. The ARMS #40 spring loaded flip up sight is a very reliable back up sight.