Don't take my word for it, I'm not expert. Just judging by the links, I'd say you probably have a casing from a DU shell that's been reloaded with a target practice slug. Odds are if someone did that, they didn't reload it with propellant to be fired again and its simply a decorative piece. If this is the case, the casing below the narrowed neck should feel empty, although there's always the possibility they simply filled it with sand or something to make it more stable standing up.
DU is heavy, but the target practice slugs would have to mimic their weight in order to be effective practice, so weight isn't really anyway to tell for sure. The firing cap, does it have a indent in the center? That would be a good sign that it's been fired.
I'd be pretty sure this isn't meant to be fired. The military is generally very good about color coding it's ammunition and weapons to prevent any mistake of what's loaded in what. They wouldn't mix the casing color from one type of shell and the slug from another.
