Don't know what kind of comments I'll get from this new-fangeled way of fixen your problem, but after roughing up the dovetail in the barrel, and roughing up the base of the sight, and cleaning them both with a little bit of alcohol, try using some Locktite. You would probably want the semi-permanant stuff, and if it just has to be removed later, it can be broken down with heat. You used a propane torch to
heat the barrel to brown it in the first place didn't you?
Or epoxy might do the same thing better, but any excess wouldn't wipe off as easily as Locktite.