The W56 is an American thermonuclear warhead produced starting in 1963 which saw service until 1993, on the Minuteman I and II ICBMs.
The W56 was manufactured in a series of models, all the same rough size and 1.2 megaton yield. The Mod-1, Mod-2, and Mod-3 variants weighed 600 pounds, with the Mod-4 variant weighing 680 pounds. All versions were 17.4 inches (440 mm) diameter by 47.3 inches (1,200 mm) long.
The early models (Mod-1, Mod-2, and Mod-3) were produced starting in 1963 and retired in September 1966. The Mod-4 model was in production through 1969 and saw service until 1993. 1,000 total were produced, of which 455 were Mod-4.
The early models had to be retrofitted to fix a reliability problem, likely leading to their early retirement.
All retired W56 warheads were stored, to be finally dismantled by the end of June, 2006. One warhead nearly experienced a high-explosive detonation with no nuclear yield in 2005 because an unsafe amount of pressure was applied while it was being disassembled [1]