No, it is not that difficult, at least if you are fine with not having the most efficient weapon. You take a metal pipe, put a lump of Uranium and some explosiv

No, it is not that difficult, at least if you are fine with not having the most efficient weapon. You take a metal pipe, put a lump of Uranium and some explosive at each end and to a first approximation you are done. The USA did not even test their gun type design before dropping it onto Hiroshima because it was so simple that they were confident that it will just work.