Gun-type fission weapons are fission-based nuclear weapons whose design assembles their fissile material into a supercritical mass by the use of the "gun" method: shooting one piece of subcritical material into another. This design is rather inefficient owing to the slowness of the process and the amount of uranium required, and has fallen out of use. The bomb Little Boy, which was detonated over Hiroshima on August 6, 1945, was of this type.Diagram: Dake, Papa Lima Whiskey, Mfield