According to the M&P manuals, +P+ ammo MUST not be used in S&W firearms. Ive never used +P+ ammo before, but Ive put about 50 +P ammo through my M&P. Here is the link. Page 13
http://www.smith-wesson.com/wcsstore/SmWes...r/MP_Manual.pdf
http://www.smith-wesson.com/wcsstore/SmWes...r/MP_Manual.pdf