The S&W M&P has a slightly slower rifling twist rate than most .40 pistols on the market. This increases barrel life and reduces recoil, but it makes it a little less accurate with certain grain weights and loads. My suggestion is to get a box of 155gr, 165gr, and 180gr Speer Gold Dots and see how your shot groupings are at 15yrds. All will perform well against a person, but you should pick the one that shoots the best out of the pistol. If they all shoot equally as well, go with the 180gr which is a little easier to control and puts less wear and tear on the pistol.