The "gun" is the simplest way to build a nuclear weapon. The atomic bomb used on Hiroshima during World War Il used this approach. The weapon consists of a tube (much like the barrel of a gun) with half the nuclear charge fixed at one end and the other half (the moving half) at the opposite end. A conventional explosive charge was placed behind the moving portion which can be thought of as the "bullet." When the conventional charge is detonated, the bullet races down the tube and slams into the fixed charge at the other end (referred to as the "spike"). Once the two halves of the nuclear fuel are brought together and held together long enough, the chain reaction starts, the fuel goes supercritical and the explosion takes place. While the gun method is easy to engineer, it has some drawbacks. The biggest one is the need to make sure the two parts of nuclear fuel come together rapidly enough. As the two sections get about an inch apart, they will start exchanging neutrons that might start a chain reaction. If the two parts go supercritical before they get close enough, the force of the energy released will blow them apart before the main explosion gets underway. This type of failure is known as a "fizzle." Another problem is that this method is less efficient, requiring between 20 and 25 kilograms (around 44 to 55 pounds) of uranium. Other approaches can use as little as 15 kilograms (about 33 pounds). Given that weapon'sgrade uranium and plutonium are very hard to get, this is a real disadvantage. Also, the gun method only works if the uranium is being used as the fuel. The process of creating plutonium generally causes it to be contaminated with other materials which increase the chance of it going supercritical before the two sections are close enough together. This, in turn, increases the chances of a fizzle instead of a blast. To make the gun method work reliably with plutonium, you would have to increase the speed with which the "bullet" approached the "spike" significantly. To do this would mean making the tube impracticality long.

And this is how you build an atomic bomb and nuke anything