When you say corroded, do you mean pitted? If so, there is not much you can do. It is impossible to add metal back. What I would do is to bronze brush the you know what out of it and then possibly fill it with some bore cleaner (follow label directions) and let it sit and then patch it out. More than likely, you will never ever get it clean looking or have nice clean patches come out while cleaning. With a pitted/corroded bore, you will have a hell of a time keeping it clean. Jacket fouling will be horrible, more than likely. It should be safe to shoot, but you might want a "pro" to look at it for sure. All disclaimers apply.

Pitting or frosting (very light pitting) is common in milsurps.  I have cleaned up the barrles so they are free of all obstructions and then gone out and shot several rounds through them.  The nclean immediately.  A lot more stuff should come out.  I have been able to even restore some shine to some of mine.  I used to ahve a Krag with a real sewer pipe bore.  Shot great.  Also, if the barrel is priited a bit, expect to take a little extra time cleaning it out.