he simplest type of nuclear weapon is a "gun-type" fission weapon. A gun-type weapon uses chemical explosives to "shoot" one subcritical mass of HEU into another at high speed, much as a bullet is shot from a gun. When the two masses collide, they form a supercritical mass which produces a nuclear explosion.

Gun-type weapons are the easiest type of nuclear weapon to design and manufacture, but typically require more fissile material than implosion designs.  Depending upon the sophistication of the design and the skills of the bomb makers, at least 25 to 50 kg of HEU enriched to 90 percent U-235 would be needed to build a gun-type nuclear explosive. The "Little Boy" bomb used against Hiroshima on August 6, 1945 was a gun-type fission weapon with an explosive yield of about 13 kilotons. South Africa built six gun-type weapons starting in 1979, but destroyed all of them by 1993.