For full-size 9mm, .40 S&W and .357 SIG pistols. Factory spring is 16 lbs. Use a lighter-weight spring with light loads or to soften felt recoil for faster recovery in competition. Use a heavier-weight spring with “hot” loads or to enhance cycling reliability. NOTE: These springs require a full-length, non-captive guide rod. Each recoil spring, including those in calibration kits, comes with a factory weight striker spring. Calibration Paks let you try out different weights to see which works best in your pistol. Extra Power (XP) Recoil Calibration Pak contains one each 17, 19, 20, 22, and 24 lb. recoil spring, plus five replacement, factory-weight striker springs. Reduced Power (RP) Recoil Calibration Pak contains one each 12, 14, and 15 lb. reduced-power recoil spring, plus three factory weight striker springs.
Big Book Catalog, Issue:65, Page:211