The first nuclear bomb ever used in combat, "Little Boy" dropped on Hiroshima, was of a design that was previously not tested, a gun-assembly Uranium bomb. Pret

The first nuclear bomb ever used in combat, "Little Boy" dropped on Hiroshima, was of a design that was previously not tested, a gun-assembly Uranium bomb. Pretty much anyone with modern off-the-shelf machine tools and enough HEU (which is the tricky part) could make such a bomb fairly easily.