Your post is not specific on your experience with the AR platform. At the risk of insulting you, and I truly am not intending to do so, installing a free float rail is not simply a crank off the barrel nut and slide it on. You will likely have to remove the front sight post/gas block and gas tube. On the free float barrel nut they sometimes require shims to fit it all up properly. All part of the fun of tinkering with the AR.
“Only at the end do you realize the power of the Dark Side.”