The 31-round .40 S&W magazine for GlockA(R) from KCI is built to shrug off hard use and maintain reliable feeding, round after round. Constructed with a reinforced polymer outer sleeve and a stainless steel liner for extra support, just like factory mags, KCI's magazine will fit both G22 and G23 pistols. KCI's mag spring is made of tempered chrome silicon steel wire. Like many extended-capacity magazines, it needs to be loaded/unloaded several times with .40 S&W ammo before it will accept its full 31-round capacity. Try loading it to half capacity and leaving overnight to help set up the spring and make it easier to insert a full load later. Polymer body Mfg: Kci Usa