For optimum reliability with hardball and hot handloads. Improves overall function by matching the spring to the specific loads being fired. 13lb., factory standard weight, recoil spring only. Recoil Calibration Pak: – 1 each 15 lb., 16 lb., 18 lb., and 20 lb. extra power recoil springs, plus (3) extra power firing pin springs. Performance Pak: – 1 each 10 lb. reduced power, and (1) 15 lb. extra power recoil spring, plus (1) extra power, firing pin spring and (1) 18 lb. reduced power hammer spring.