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?