Increase slide speed, lessen felt recoil, and change the way the gun reacts during recoil.
The correct spring and ammo configurations will create a gun that shoots flatter, and faster.
We recommend the 15 lbs spring for major PF .40 s&w. For 9mm or 40 minor we recommend the 13 lbs spring. For major 9 open Glock will depend on your load, and gun configuration.
The stock Glock recoil spring is 17 lbs. Generally a aftermarket guide rod is required for these springs.