I'm getting ready to have an action/trigger job done on my girlfriend's S&W 640. Any reccomendations on Wolff Springs to use? They offer hammer springs in 8, 8.5 & 9 pounds and rebound springs in 11,12,13,14,15 & 16 pounds. Wolff says the factory rebound spring is 18 pounds. No word on the factory hammer spring weight. This is for a carry gun so absolute reliability is essential. Thanks!