There is a tool for it, I think it's about $40 but it's easy enough to make your own. I used electrical conduit pipe, ground to fit the forearm nut slots then drill a hole through the pipe to insert a screw driver to use as a "T" handle.
h t t p ://ww w.cabelas.ca/product/30424/ati-mag-socket-wrench-forend-removal-tool