 Following Oppenheimer's reorganization of the Los Alamos Laboratory in July 1944, the work on the uranium gun-type weapon was concentrated in Francis Birch's O-1 (Gun) Group.The concept was pursued so that in case of a failure to develop an implosion bomb, at least the enriched uranium could be used.Henceforth the gun-type had to work with enriched uranium only, and this allowed the Thin Man design to be greatly simplified.