How do gun-assembly and implosion atomic bombs differ?

Atomic bombs achieve a nuclear explosion by rapidly assembling subcritical masses of fissionable material into a supercritical configuration. The two primary methods for this assembly differ significantly in their mechanics and material requirements.1

Gun-Assembly Method

In a gun-assembly device, an explosive propellant fires one subcritical mass down a barrel into another subcritical mass. This method is limited to highly enriched uranium-235. It cannot be used with plutonium because the assembly speed is too slow; spontaneous neutron emission from plutonium-240 would cause the device to "pre-initiate," resulting in a premature, low-yield explosion.123

Implosion Method

The implosion method uses chemical high explosives to compress a subcritical mass of fissile material—such as plutonium or uranium—into a denser, supercritical state. This process is faster and more efficient than the gun method, requiring significantly less fissile material to achieve a comparable yield. Because it compresses the core inward, it avoids the pre-detonation issues associated with plutonium, making it the necessary design for plutonium-based weapons.123