gsxraddict got it right, it's a mortar round. When I was a stupid teenager we used to sneak on to the impact area at a local army base, and pick up stuff like that. You might have a "dud" round... dangerous, in other words. If you have an acquaintance that is familiar with ordnance, you should have them look at it. But then again, maybe you should call the local sheriffs department and let them know what you found. They will probably call in EOD (Explosive Ordnance Disposal) people who will blow it up.