Looks like with your M&P15x you can pull out the drop in rail for traditional plastic hand guards, this is will be the simplest and cheapest route to shave some weight. If you wanted to go with a free floating hand guard like the precision reflex, most will require a barrel nut change with the manufacturer's proprietary parts. Appears the m&p15x is a carbine gas system, which would limit you to shorter hand guards if you were wanting to keep the front sight base. Another option is cutting down the FSB or installing a low profile gas block, so you can have whatever length and make of hand guard your heart would desire.