Install the RIS on the nut that way.
Then use a screw driver that fits in one of the holes in the rail (There is 2 on each side if I remember correctly.
That will give you enough torque to remove the barrel nut.
Then, either buy this:
http://madbull.com/catalog/en/produc...ut-for-we.html
Or rethread the upper recevier to use the metric nuts.
That implies buying the correct die if you don't have it or find a gundoc that has it.