No clue but it's certainly an interesting find. Keep checking in; no-doubt somebody knows. I'm curious now.
If you could get actual measurements (i.e. length and diameter) and tell us where you found it that might help in the ID.
It's quite possible that it's not a bullet (doesn't really look like one and it's definitely not a shell) but, rather, part of something decorative (e.g. the end of an umbrella, say) or functional.