Gun-type fission weapon

Gun-type fission weapons are the simplest approach and they work with Uranium-235 bombs as you can ignite it with just one explosion.

But Gun-type fission weapons don't work with plutonium, and weapon grade Plutonium is cheaper than weapon grade Uranium, so it wasn't much used.

Gun-type fission weapons don't work with Plutonium-239 because of the presence of Plutonium-240 as an impurity which leads to fizzle.

Good mentions at: youtu.be/dgBDvnqMkT4?t=252