Tacticool22 makes an adapter that you can put on your S&W M&P15-22 so you can use any normal AR-15 free float handguard. The adapter is only $27.

They even have a faux gas block so you can use two piece handguards if you so desired.

Here bret248vista modified his S&W M&P15-22 into a M4 Carbine style.

Tacticool22 also has an entire kit that they have assembled for you to uninstall the factory handguard and to install their adapter. That retails for $79. There are two versions. A standard kit and a modified kit. The difference is how much of a gap you want between your handguard the freefloat. The standard kit will have a 1/4″ gap. The modified kit will only have a 0.1″ gap between the top rail and handguard.