Interesting read regarding M&P9s and bullet weights from Hilton Yam's blog. I have yet to test this myself... probably will sometime this week though and then I'll let yous guys know if I found 147 grains to be any better. I have always felt the 115 and 124s are rather inaccurate out of my M&P... it's too bad those are the cheapest.
"The main downfall of the M&P 9mm is the wildly varying accuracy thanks to some QC issues and design flaws. The M&P 9 barrel uses a tragically slow 1:18.75" twist, which does a poor job stabilizing heavier bullets. To add insult to injury, the M&P also unlocks extremely quickly. This early unlocking tends to negate any conventional wisdom on what bullet weights should shoot well with the M&P, and you'll want to do a bit of experimenting. The Glock is inherently more accurate than the M&P, and will shoot 3" or better at 25 yards with a wide range of ammunition. I feel that 3" or less is a good measure for service pistol accuracy at 25 yards. My 3 primary M&P 9mm's will all shoot 3" or better at 25 yards with any 147 grain ammunition that I feed them, and two of them have printed 2 5/8" standing offhand at 25 yards with 147 grain. That works for me. However, all three of the guns shoot about 6-8" at 25 yards using 115 and 124 grain ammo. My solution at this point is just to stick with 147 grain. "