I have a quad rail free float tube, adn it rotates about 1/8" back and forth. It feels like it is hitting a stop when roatated left or right. Any idea how to fix this???

Ryan

What kind is it ? Does it have an end cap? FVG? We need more info !  

If your barrel nut is loose it could be hitting the gas tube or a end cap in the front of the tube.

           Invisiblesoul

Ditto,

Sounds like the barrel nut section of the tube was not tightened correctly, and the only thing preventing the hand guard from completely spinning off is the gas tube that goes threw it (one of the barrel nuts index channels). Either that or you have one of the standard guard replacement types, and slight rotation on these units is normal (same as a standard hand guard loosing up over time).

If you want to give us a type, we can walk you threw tightening it back up if possible, or if you don't have tools or is not a adjustable unit, then it may be time to take/send it back to be corrected/replaced with a solid type unit if the movement bothers you.