Only thing that can happen is carbon build up. That stuff gets hard as a rock, and you can have layers of carbon and copper.

Some have said, and I tend to agree, that the byproducts of powder residue can eat steel. This is in extreme cases I beleive where the gun is shot and shot and never cleaned. But I have seen pitted bores that are not rusted but shot alot with a huge carbon/copper buildup.

I go too crazy cleaning copper myself. I get out every last bit. I have actually damaged a throat a long time ago over doing it. Can't seem to control myself.