The extension will look kind of silly, sticking beyond the barrel, and it'll get tons of powder burns. Whether it'll be "safe" depends on how much you use it, and how many rounds you give it at any one time. I'm pretty sure a few hundred rounds in one afternoon, would do some pretty severe damage. On the other hand, if it's carried more than shot, I doubt a round or three at a time wouldn't do much worse than cosmetic damage. Of course, if you weren't planning on blasting away, you'd probably not be looking to add an extension for more rounds.