Like Geocap said, they come from the factory very tight. Also, those little hook wrenches that come with the gun aren't very good. I would order one from Cole or one of these: http://www.jassllc.us/Wonder/wondersocket.htm
I like the Wondersocket better, but you do have to have a 1/4" drive ratchet (they are not expensive). BTW, there is another product also called Wonder Socket, but this one is made especially for a 391.
If you don't want to wait for the wrench to come, use a pair of pliers, being careful to get it only on the top nut, not both nuts at one time. Try to get one of the factory wrenches on the bottom nut, and let it get jammed against the barrel so it won't turn while you turn the top nut with the pliers. It will help if you can put the barrel in a padded vise and get someone to hold the wrench on the bottom nut while you turn the top one.