fission weapons, and thermonuclear weapons. New nuclear weapon states tend to develop boosted fission and thermonuclear weapons only. All known gun-type nuclear weapons previously built worldwide have been dismantled. Little Boy The "gun" method is roughly how the Little Boy weapon, which was detonated over Hiroshima, worked, using uranium-235 as its fissile material. In the Little Boy design, the U-235 "bullet" had a mass of around , and it was long, with a diameter of . The hollow cylindrical shape made it subcritical. It was powered by a cordite charge. The uranium target spike was about . Both the bullet and the target consisted of multiple rings stacked together. The use of "rings" had two advantages: it allowed the larger bullet to confidently remain subcritical (the hollow column served to keep the material from having too much contact with other material), and it allowed sub-critical assemblies to be tested using the same bullet but with just one ring. The barrel had an inside diameter of . Its length was , which allowed the bullet to accelerate to its final speed of about before coming into contact with the target. When the bullet is at a distance of , the combination becomes critical. This means that some free neutrons may cause the chain reaction to take place before the material could be fully joined (see nuclear chain reaction). Typically the chain reaction takes less than 1 μs (100 shakes), during which time the bullet travels only 0.3 mm. Although the chain reaction is slower when the supercriticality is low, it still happens in a time so short that the bullet hardly moves in that time. This could cause a fizzle, a predetonation which would blow the material apart before creating much of an explosion. Thus, it is important that the frequency at which free neutrons occur is kept low, compared with the assembly time from this point. This also means that the speed of the projectile must be sufficiently high; its speed can be increased but this requires a longer and heavier barrel, or a higher pressure of the propellant gas for greater acceleration of the bullet subcritical mass. In the case of Little Boy, the 20% U-238 in the uranium had 70 spontaneous fissions per second. With the fissionable material in a supercritical state, each gave a large probability of detonation: each fission creates on average 2.52 neutrons, which each