The XS Big Dot tritium night sight is a superior defensive sight. It features a Big Dot Tritium front sight for fast sight acquisition in all light conditions. Big Dot demands your attention - even in high stress and for those with poor eyesight, aids in rapidly achieving combat effective hits. White dot reflects ambient light for best visibility in low light. Day-Light, Half-Light, or Low-Light, place the bottom of the dot in the bottom of the sight channel and center the dot over your target within 15 yards. Outside Diameter Big Dot 0.188"; fits in common holsters. Installation requires a drill press to drill the retaining pin hole in sight body. This model fits S&W J-Frame with pinned front sight and Ruger SP101 with a fixed rear.