Reduced-power replacement for factory striker (firing pin) spring lightens and smoothes trigger pull for competition. M & P - Fits 9mm, .40 S&W, and .357 Sig. 4 lb. rating; factory weight is 5 lb. Sigma - Fits all models except .380. 3.5 lb. rating; factory weight is 4.75 lb.
Big Book Catalog, Issue:66, Page:218