The super low-provile FRS Flip-Up Rear Sight fits a Picatinny rail and features two operator selectable apertures, including the standard military A2 style double peep aperture with leaf sight aperture. Large aperture is designed to be used for CQB low-light conditions. The smaller aperture is used for long range target engagement. Easy to reach spring-loaded button raises the sight. Rugged flip-up mechanism is designed to ensure constant battery action and repeat zero. No gunsmithing required to install or remove. Lifetime warranty.