Chogy
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Fresh from Wikipedia!
Interestingly, for decades the thought was that the Little Boy bomb fired a solid slug of Uranium into a target with a cavity. It was only recently that the diagram we see was somehow made public, and that the projectile was Uranium rings, and the target was a shaft of U-235. Supposedly, this diagram is accurate.
The Manhattan Project scientists originally thought that they could make a plutonium gun device like Little Boy, but tests showed that it wouldn't work... the rate of Pu fission is too fast, and if you could imagine ultra-slow-motion of the joining of the Pu critical mass, what would happen is that the fissile core would burst apart in the first few millimeters of joining, even with that ring traveling at cannon velocities. So it would fall apart before the mass became truly supercritical. You'd get a very dirty fizzle.
The implosion method was brilliant but hard in reality to execute. The conventional high explosive mold sections have to be perfect, and more importantly, the wiring and the detonators that lead to each section (like a soccer ball cover) each have to function at exactly the same time. If one portion of the HE detonates a fraction before the others, no implosion.
For this reason, the special cables and detonators used for these bombs are as controlled as the fissile material itself.
