Sounds like we're talking about stocks, not hand guards. Buffer tubes for ARs are either commercial spec or mil-spec. Handguards are measured by gas tube length. The M&P Sport is a carbine length as opposed to mid length or rifle length.
The title says hand guard, but since commercial vs. mil-spec pertains to a stock and not a hand guard, I made the assumption that he is talking about a stock.
Bought a MagPul hand guard to replace the cheapo one that came with my Sport - $25 at Academy.
The product number on the box is MAG440-BLK. It is the MOE Hand Guard - Carbine, and has the built-in heat shield: