I cannot speak on the M&P 15, as the only one in our house is a .22LR. However, I have an AR in .22LR, comprised of a Plum Crazy (polymer) lower, and a "dedicated" CMMG Upper in .22LR. This combination is fantastic. I am not sure as to how the M&P is built, but I can bet that BangBang will be on with an answer for you within a day or so.
You may Judge me all you want.
Keep the verdict to yourself.
I will give up my guns, when they pry
my cold, dead, fingers from around them.
"Make your attacker advance through a hail of bullets.
I may be killed with my own gun, but he is going to have
to beat me to death with it, because it will be empty."