Gun-Type Atomic Bombs

Gun-type atomic bombs operate by bombarding two pieces of fissionable material that are individually too small to sustain a nuclear reaction into each other at great speed. These pieces would ideally weigh less than about one pound / 0.45kg. The apparatus used to facilitate this reaction is similar to a gun barrel where the materials are fired at each other like artillery projectiles and collide inside the barrel. When the two pieces of material collide with sufficient speed, they form a supercritical mass provided that the combined mass will be enough to sustain a nuclear reaction. This is a fission reaction and uranium-235 can be used for the fissionable material. The explosive force of a 2.2 pound / 1kg bomb of this type is comparable to exploding about 17,000 tons of TNT. An atomic bomb of this type, named Little Boy, was exploded at 1900 feet / 579 meters altitude over Hiroshima, Japan, on 06 August 1945 at 08:15. It exploded with a force greater than the equivalent of about 15,000 tons of TNT and this was the first actual use of an atomic bomb in warfare.

Little Boy exploded at 1900 feet / 579 meters altitude over Hiroshima, Japan, at 08:15 06 August 1945. It exploded with a force greater than the equivalent of 15,000 tons of TNT. This was the first actual use of an atomic bomb in warfare.
Photo: War Department.
Office of the Chief of Engineers.
Manhattan Engineer District.
National Archives and Records Administration.
Still Picture Branch; College Park, Maryland.