WWI Trench Art. Food can or shell casing?

I would love to know if this is a rare item and whether anybody has seen anything quite like it before. It belonged to my grandfather from World War I but I know nothing more about it. Some people have told me it is the hand engraved bottom of a shell casing, but it looks more like the bottom of a can of food to me. Anything anybody can tell me or show me comparable items would be appreciated.