Jerry Stiller of Stiller Precision
has his Python action that weighs in around 42oz. I would double check with Jerry to make sure he recommends the Python for the .590 bolt face. And right along that same weight is Bruce Thom's BAT Machine Co, Inc
M Model action 8.5"x1.47" multi-flats. All of the other custom BR action makers that I'm aware of are heavier than those mentioned above. There may be others that are lighter but I'm not aware of them. I would again double check with BAt to make sure the M model will handle the .590 bolt face also. It has 1 1/8" diameter threads.
Also these are all custom built actions that are single shots and the ejection port probably won't handle ejecting a loaded round either. Your going to have to give up something to maintain a lightweight rifle.
[ 03-25-2002: Message edited by: Steve Shelp ]