text: For technologically advanced states the gun-type method is now essentially obsolete, for reasons of efficiency and safety (discussed above). 
  The gun type method was largely abandoned by the United States as soon as the implosion technique was perfected, though it was retained in the specialised role of nuclear artillery for a time. Other nuclear powers, such as the United Kingdom, and the Soviet Union never built an example of this type of weapon. Besides requiring the use of highly enriched U-235, the technique has other severe limitations. The implosion technique is much better suited to the various methods employed to reduce the weight of the weapon and increase the proportion of material which fissions. South Africa built around five gun-type weapons, and no implosion-type weapons. They later abandoned their nuclear weapon program altogether.  They were unique in their abandonment of nuclear weapons, and probably also by building gun-type weapons rather than implosion-type weapons.
  There are also safety problems with gun-type weapons. For example, it is inherently dangerous to have a weapon containing a quantity and shape of fissile material that can form a critical mass through a relatively simple accident. Furthermore, if the weapon is dropped from an aircraft into the sea, then the moderating effect of the seawater can also cause a criticality accident without the weapon even being physically damaged. Neither can happen with an implosion-type weapon, since there is normally insufficient fissile material to form a critical mass without the correct detonation of the explosive lenses.