If and when I carry my M&P FS9, I use a Little FoxX IWB holster. Love the gun...love the holster. Fits my body great at 4 o'clock. Having said that, I don't like to carry that gun for very long. It's a tad big even thought I'm 6'4" and about 210 lbs. When I carry, it's only for a half a day or so in the fall and winter months. That's why most of the time I carry a 9mm Shield. But that's just me. Good luck finding what works for you.

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M&P FS 9mm
M&P Shield 9mm