The XS Standard Dot tritium night sight is a superior defensive sight. Fits S&W J-Frame with pinned front sight and Ruger SP101 with a fixed rear. Precision machined in Fort Worth" TX" XS Sights feature a Standard Dot Tritium front sight for fast sight acquisition in all light conditions. Standard Dot is larger than most fronts to be seen under stress" but covers less of the target than the Big Dot. White dot reflects ambient light for 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 Standard Dot 0.140"; fits in common holsters. Installation requires a drill press to drill the retaining pin hole in sight body.