Gun-type fission weapon

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 unless it is purely the 239 isotope. Production of impurity-free plutonium is very difficult and is impractical. The required amount of uranium is relatively large, and thus the overall efficiency is relatively low.

The method was applied in four known programs. First, the "Little Boy" weapon which was detonated over Hiroshima and several additional units of the same design prepared after World War II, in 40 Mark 8 bombs, and their replacement, 40 Mark 11 bombs. Both the Mark 8 and Mark 11 designs were intended for use as earth-penetrating bombs (see nuclear bunker buster), for which the gun-type method was preferred for a time by designers who were less than certain that early implosion-type weapons would successfully detonate following an impact. The second program was a family of 11-inch nuclear artillery shells, the W9 and its derivative W19, plus a repackaged W19 in a 16-inch shell for US Navy battleships, the W23. The third family was an 8-inch artillery shell, the W33. Later, South Africa also developed six nuclear bombs based on the gun-type principle, and was working on missile warheads using the same basic design – See South Africa and weapons of mass destruction.

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