Factory replacement parts are manufactured to the exact same specifications and tolerances and use the same manufacturing materials as the original parts. Some parts may require installation by a qualified gunsmith. This pack contains 1 each of 16, 17, 18 and 19 pound reduced power hammer springs above, allowing adjustment of the firearm to individual requirements. Note: Recently Sig has started using a plastic hammer spring base/retainer with a short hammer. spring. This is being phased in across all models. If your pistol has the plastic base/retainer for the hammer spring you will need to select an option from product family # 3415118865.