https://en.wikipedia.org/wiki/Gun-type_fission_weapon https://en.wikipedia.org/wiki/Gun-type_fission_weapon Look at the diagram on the right - it's the cylinder

https://en.wikipedia.org/wiki/Gun-type_fission_weapon https://en.wikipedia.org/wiki/Gun-type_fission_weapon Look at the diagram on the right - it's the cylinder that's the target.