Some interesting stuff about my 21SF. I hope it helps anyone else who has found this. The 13 round magazines rattle like heck when you put 13 rounds in them. I've tried the palm slap - hard enough to hurt. I've tried everything I can think of but the rounds rattle around in the magazine like they are loose and not being held under spring tension. If I load 12 rounds into the mag they do not make noise... until I put the mag into the gun. The extra pressure of the top round hitting the slide makes it rattle when I walk. Not good. When I put 13 rounds in the mag and then put it into the gun... guess what? The extra pressure on the top round then makes it stop rattling. Glock perfection hey? Wanna kill these ads? We can help!