Why could plutonium not be used in the gun-assembly design?

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Atomic bomb — atomic bomb The first atomic bomb test, near Alamogordo, New Mexico, July 16, 1945.

Plutonium cannot be used in a gun-assembly design because the assembly process is too slow to prevent the high probability of "pre-initiation" caused by spontaneous neutron emission. In this design, the speed at which the subcritical masses are joined is insufficient to preclude a chain reaction from starting prematurely, which would result in a significantly reduced explosive yield of only a few tens of tons.1

This issue is exacerbated by the presence of plutonium-240, a common by-product in plutonium production that exhibits a high rate of spontaneous fission. Because of these limitations, fission weapons utilizing plutonium must instead use an implosion method, which compresses the fissile material into a supercritical mass much more rapidly.231