I think what your saying is that someone told you NOT to run 5.56 ammo in the M&P as it will wear your bolt prematurely. I would NOT be inclined to agree with that personally. If you buy a rifle thats chambered for 5.56 pressures and your not shooting full-auto i wouldnt worry about the bolt at all regardless of the ammo you are shooting.I have a smity mp nato love it never a hiccup over 2000 round,s.yet i was advised to keep 223 vs more powerful rounds for the bolt may give up up the ghost.IM inclined to agree im blessed to have the best around me.
I have a Smith MP...its a great rifle. Never given me a bit of trouble.