My recommendation would be to buy a few boxes of different weight pand brands of ammo that are on the FBI approved list. Shoot them through your gun and see which is the most accurate and/or which you like the most. That being said I'm leaning toward the lighter faster flatter shooting ammo for personal reasons. The FBI testing results are pretty easy to find and fairly straightforward to understand. You will be well served with anything that "passed". Accuracy and reliability out of your gun are more important than the exact brand or weight. Most are so close they just have small trade offs like this one does slightly better through drywall, but this one does better through heavy clothing.

Side note, the M&P seems to mitigate the +p+ recoil better than a glock (to me anyway), but if your recoil sensitive, that can be a concern as well.

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