“I found this in my grandmother’s apartment. Is this a deactivated bomb or something?”
It looks like what you have there is a WWII era 81mm mortar. There is no fuse in the nose of it, and by all accounts, it should be inert. However! If you look down the hole that should be in the nose and it’s filled with a plastic-y, somewhat smelly brownish material, IT COULD STILL BE LIVE.