This flatwire recoil spring allows the user to tune their pistol to their specific ammo. The Walther pistols arrive oversprung from the factory. 13lb and 11lb ISMI springs can provide reduced muzzle flip when paired with weaker power factor loads. We will NOT advise or recommend spring weights based on ammo. It is at the user’s own RISK. NOT for use with +P or +P+ ammo.