I have a similar handguard made by Barnes and the hex-head screws holding it on the barrel nut have opposing and offset screw holes that are used to spread the handguard apart for installation/removal. Just remove the hex screws and thread them into the opposite side to make the handguard bigger than the barrel nut. http://www.usamade-ar15parts.com/v/vspfiles/templates/BPM_sales/images/BPM-Manual.pdf
They're not all the same, of course, just something to think about next time.