Various weights of reduced power hammer springs in one kit help you precisely tune your semi-auto pistol for the smoothest double- and/or single-action pull and proper function. KIT #26581 S&W 39/59, 439/459, 3904/3906, 5903/5904/5906, 645/745/4506, 4006/1006 - Contains 1 each 16 lb., 17 lb. and 18 lb. hammer springs. Factory spring is 20 lbs.