Mark 4 (nuclear weapon)/Definition

Still first-generation but a production-quality, reengineered version of the FAT MAN bomb, the yield of which could be varied from 1, 3.5, 8, 14, 21, 22, and 31 KT by exchanging the plutonium pits; first weapon made on an assembly line rather than by hand; design ancestor of the British Blue Danube bomb