The ML18801 is a fixed set for use on the Walther P-99 9mm and .40 Compact Pistols and .45 Full Size Pistols. 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..