The ML22771 is an adjustable set for use on the Smith and Wesson K,L and N frame revolvers that have a pin-on front sight and fully adjustable rear sight. This model is not meant for use on the "Mountain Guns". Mountain guns have a much higher front sight. MEPROLIGHT sights, like our other self-illuminated products & devices take maximum advantage of tritium light sources - the fail-safe, maintenance-free light sources that don’t require any external source of energy and provides up to 15 years of usable light under all weather conditions. MEPROLIGHT continues to offer the strongest guarantee in the industry for the usable light of our TRU-DOT sights.