Knights Armament Company Rail Mounted Anti-Cant Device Fits Picatinny Black Finish 30855 The KAC Rail Mounted Anti-Cant Device is a simple, low profile tool that helps precision shooters ensure that their rifle is level before breaking a shot It incorporates a small bubble level into an aluminum base that sits low enough to permit its use under most optics Attaches to any M1913 Picatinny rail surface via a screw and clamp arrangement Made In The USA