I have a theory about the rattle. I believe it is caused by the rounds of ammo in the magazine not seating properly against each other allowing some wiggle room if you will. I have a 9mm Kel-Tec P-11 with magazines that sometimes rattle. I find that if I give the side of the full magazine a good rap or two against the palm of my hand the rounds seat themselves and no more rattle. By the way, my M&P 40 magazines do not rattle... or have not with the ammo I have used so far.

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Life Member B.A.S.S., NAFC