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 sub-critical material into another. Although this is sometimes pictured as two sub-critical hemispheres driven together to make a supercritical sphere, typically a hollow projectile is shot onto a spike which fills the hole in its center. Its name is a reference to the fact that it is shooting the material through an artillery barrel as if it were a projectile. Other potential arrangements may include firing two pieces into each other simultaneously, though whether this approach has been used in actual weapons designs is unknown. Since it is a relatively slow method of assembly, plutonium cannot be used practically (see below). The required amount of uranium is relatively large, and the efficiency relatively low. The method was applied in the "Little Boy" weapon which was detonated over Hiroshima, and also in 40 Mark 8 bombs, and their replacement, 40 Mark 11 bombs.