Model of 20x102 mm Vulcan round, used in M61 Vulcan or M197 Gatling for example. Can be used in training, as movie prop, cool gadget (in advertising for example), an ornament or simply your gun store decoration.
Shell is hollow inside, so cartridge can be used as container for small things, a gift (You can hide money in there, if You don't have an gift idea. And honestly, is there any more cool gift than 20 mm Vulcan round with money inside?) Shell also can be optional customized! You can even wirite somebody's name on it, and place it under Christmas tree!
Version III. Bullet cap and shell have threads.