Best case scenario from calling 911 is the bomb squad treats it as they would a terrorist's bomb and sends in the robots to blow it up in your home or office, then bring charges against you for keeping it. They are not usually experts on dealing with old live ordinance. Just calm down, leave it lie and see who in your area specializes in dealing with old ordinance. In the case I know of it was a special air force unit. They were able to safely transport the 1890's vintage shell away from town, destroy the detonator and remove the explosive. They knew how the shell operated and were able to make a realistic conclusion about its potential threat, rather than freaking out.
When you call 911 and say you HAVE A BOMB(!!!!), you're unleashing the hounds and spreading red meat around for them. That number is only supposed to be used in active emergencies Like when someone is holding a live shell to rob a bank. Not when you realize the shell that's been on your bookshelf for ten years might possibly still be live ordinance. Unless your local authorities have instructed people to call 911 for old ordinance, I would not advise calling it up and saying you have a bomb.