I've had mixed luck with barrels. I wouldn't do one again myself. I only buy CM actions and I have them done and it is a great treatment for such. I have had NO rust on any of them that I have had done.
I have put my Glocks through hell with nary a trace of rust. I suspect that Salt bath nitride on a barrel could be fine but I think you need to have it perfect before you treat it and I have had 2 brand new barrels that didn't shoot very good after being treated. My guess is to break in the throat very thoroughly first and then clean the dickens out of it before sending it off. I'm not willing to spend $600 again to try.
