The only free-float I have (I'll be changing my carbine in the future as well) is on a short pistol, but like Les, there is a good 1.5 inches of fine threads...and mine has a locking ring and an extended set-screw that locks into the grooves in the center of the nut. It is very rigid and I don't think that would change if it were any longer.
I paid $30 for the kit that had the nut, fore-end, and four keymod rails with an end cover. I will probably buy the same version, just longer, for my carbine.