 Oppenheimer , reviewing his options in early 1943 , gave priority to the gun @-@ type weapon , but as a hedge against the threat of pre @-@ detonation , he created the E @-@ 5 Group at the Los Alamos Laboratory under Seth Neddermeyer to investigate implosion . Implosion @-@ type bombs were determined to be significantly more efficient in terms of explosive yield per unit mass of fissile material in the bomb , because compressed fissile materials react more rapidly and therefore more completely . Nonetheless , it was decided that the plutonium gun would receive the bulk of the research effort , since it was the project with the least amount of uncertainty involved . It was assumed that the uranium gun @-@ type bomb could be easily adapted from it . 
