Fat Man atomic bomb Standard Apron

The entire explosive assembly had to be made to very precise tolerances. The composition and densities of the explosives had to be accurately controlled and extremely uniform. The pieces had to fit together with an accuracy of one-thirty second of an inch (0.8 mm) to prevent irregularities in the shock wave. Accurate alignment of the lens surfaces was even more important than a close fit. A great deal of tissue and blotting paper and adhesive tape was also used to make everything fit snugly together with no air gaps. http://nuclearweaponarchive.org/Nwfaq/Nfaq8.html#nfaq8.1.1