Mark II: February 5, 1939 - August 24, 1943
"The dense mass of the highly compressed cylinder walls and cylinder ends confined the active material for the brief fraction of a second necessary for the initiation of a nuclear fission chain reaction by means of a neutron source placed within the active, and thenceforth propagation of an explosive fission chain reaction.
The high explosive which encased the Mark II cylinder was itself confined by a casing of depleted uranium or lead beneath an outer cylinder of tensile steel, which collectively acted as a tamper. In total the Mark II weighed approximately 1,120 pounds (510 kg). Navy Captain William S. Parsons said the process of imploding a cylinder capable of momentarily containing an evolving fission chain reaction would be "like trying to squash a full can of beer without ejecting any of the beer." The Mark II was that theoretical can of beer. No illustration of the actual construction of the Mark II is available in the declassified literature. The Mark II was essentially a nuclear fission pipe bomb."