The Mark 4 nuclear bomb was an American nuclear bomb design produced starting in 1949 and in use until 1953. 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 design was essentially handmade and designed as an emergency wartime expedient design; the Mark 4 utilized essentially the same basic design (materials, dimensions of the nuclear core and explosive components) but reengineered the whole design to be safer and easier to produce. The basic ideal was to "GI-proof" otherwise sensitive nuclear weapons.