The most basic type of nuclear bomb uses uranium. Only uranium 235 consistently gives off neutrons of an energy that will cause other uranium atoms to split. Putting enough uranium together will start a chain reaction starts . This is not enough to create a bomb. The material must be held in a compressed state long enough for the reaction to take place while resisting the initial energy of the explosion that will try and tear it apart. The gun design – the simplest – uses a piece of fissile uranium fired at a fissile uranium target at the end of the weapon, achievied critical mass when combined.
While the gun method is simple, it is subject to a problem known as 'fizzle', where uranium does not come together quickly enough. If this happens the energy released will blow them apart before the main explosion gets underway.
The gun method also requires more uranium than other methods – maybe three times as much. Uranium 235 is rare and expensive to purify, so this is a real problem. Plutonium, though it can give rise to a chain reaction is not suitable because 'fizzle' is much more of a problem. To cure it, the barrel would have to be very long, which is impractical.