I can't speak from personal experience, but i've been eyeing the Joab horizontal mag pounch by Dale Fricke for a while. There's a review on here about with pix and it looks great.
Thanks for the suggestions! Kytex looks like it might just fit the bill!Kytex and PJ Holster are both very low profile. They don't work as well with slick metal mags like standard M&P full size mags but I've had good luck with plastic covered mags like Glock and rougher metal mags like the M&P Shield.
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