Louis
At First thought, everybody would say, "that is ridiculous". But, if you have ever glued two properly prepared metal sufaces together with JB Weld, and then saw just what it took to get them apart, you might think the joint was stronger than four 6-40 screws.
By properly prepared surface, I mean the bottom of the scope rail blasted with nothing finer than garnet, and the same for the top of the action after you have masked off the surface.. (in your initial post, you said barrel, I assume you meant action). Try to do the assembly emmediatly after blasting, any thing you do to it after will only compromise things.
A Rifle such as a typical 6PPC has a pretty light recoil, there is a good possibility that a scope rail would stay put if properly glued.' Anything over that, I would say not.
You must have a reason for wanting to do this. It would seem like an easy way to mount a rail if there was not another option. If you did try it, the main emphasis is on the proper preparation of the two mating surfaces.
As for glue, any 3000 pound epoxie would probably work, JB Weld is still hard to beat.
Tell you what. I'm not going to try it, but if you do, let us know how it works........jackie