The Kriss Glock 21, 21SF magazine extension kit will add 12 rounds to your factory Glock magazine for a 25+ round total capacity. The extension consists of a sleeve, base plate and extended magazine spring. The factory follower is used with this extension kit. The back of the base pad extension has round count witness holes to keep track of your ammo. This kit works best with newer magazine bodies, pre-ban magazines may have reliability issues.