can anyone tell me what is the benefits of a free float tube?

From what I've gathered....a little better accuracy when shooting like 500+ yards. If you do alot of prairie dog shootin it might be a good idea to have a free floater. Again, thats just from what I've read thats my take on it.

Free floating a barrel does improve accuracy by allowing the "whip" to be un-affected by any items.  However, I don't have any experience on it's benefits on an AR.

Free float tubes are commonly used for NRA Highpower and other forms of competition.  A GI-spec rifle has the handguards and sling attached to the barrel at the front sight block; any pressure applied to the sling or handguards will be transferred to the barrel and deflect it.  It is a larger problem for light barrels, but is readily detectable with match barrels 1" in diameter.  A float tube is attached to the barrel nut, and the sling is attached to the tube so pressure isn't transmitted to the barrel.  Whether to use a float tube depends on your requirements for weight and accuracy.  A properly-prepared match AR will shoot sub-MOA groups as far as 1000 yards.  John Feamster even used one in a benchrest match and shot an "official screamer" - a 1/4" group at 200 yards IIRC.  On the other hand, if you're using it as a 25-yard tactical/defense weapon you won't need that kind of accuracy and the tube would be irrelevant.

Thanks to you guys for the response. been reading the forum and the float tube keeps coming up.I been trying to decide what kind of rifle to build, I like the CAR 16" and the standard 20"and I like the flattops. The longest range I"ll be shooting is about 100-200 yards and it will be just for plunking, no serious stuff. I'll be using a Fab10 lower receiver to build on.