You could try some of that loctite thread repair compound.
http://www.henkelna.com/product-sear...=8797876355073
Or attempt to drill/tap a set screw into the top side of the ris, get the barrel tightened down with the nut, get the ris sorta threaded on with the existing threads, position it and tighten set screw at 12:00 to pinch/lock it in place...