The simplest and first developed method to combine two subcritical masses is to simply employ a "gun" that fires one mass into the other, creating a critical mass for an uncontrolled chain reaction. This method can only be used for uranium-235. Plutonium-239 would be likely to detonate prematurely due to its higher spontaneous neutron release.
For a uranium fission device, a sphere of U-235 contains the neutron generator at the center. A small bullet of U-235 is removed and placed at one end of a long tube with explosives poised to propel it into the spherical mass. When the barometric pressure indicates the correct altitude, a sensor starts the following sequence:
Little Boy, the bomb detonated over Hiroshima, had a gun-triggered design. The designers were sufficiently confident of Little Boy that they did not test it before using it in war.
Because of the relative simplicity of this design, experts say that nations or terrorist groups who acquire enough highly enriched uranium (HEU) could potentially create a rudimentary but effective gun-triggered fission bomb.
The efficiency of Little Boy and other gun-triggered devices is relatively low - 1.5%. This means that 1.5% of the uranium fissioned before the explosion blew the bomb apart.
"You get two subcritical masses of fissile material and then essentially you put them together mechanically and then you have the prospect of something that can grow uncontrollably..." - listen
Dr. Rajesh Maingi, Senior Research Scientist, Princeton Plasma Physics Laboratory
"...The simple design, what's called the gun-type design, simply slams two pieces together and creates a critical mass, and that is fairly easy to do..." - listen
Reverend Robert Moore, Executive Director, Coalition for Peace Action