The Mark 4 was based on the earlier Mark 3 Fat Man design , used in the Trinity test and the bombing of Nagasaki . The Mark 3 was essentially handmade and constituted an emergency wartime expedient design ; the Mark 4 utilized essentially the same basic principles ( materials , dimensions of the nuclear core and explosive components ) but reengineered the overarching system to be safer and easier to produce . The basic idea was to " GI-proof " otherwise sensitive nuclear weapons
Trinity test
Trinity (nuclear test)
Mark 4 nuclear bomb