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How does a gun-assembly atomic bomb bring fissionable material to critical mass?

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U.S. hydrogen bomb test in the Marshall Islands — nuclear weapon A test of a U.S. thermonuclear weapon (hydrogen bomb) at Enewetak atoll in the Marshall Islands, November 1, 1952.

A gun-assembly atomic bomb achieves a supercritical state by using an explosive propellant to fire one subcritical mass of fissionable material down a "gun barrel" into another subcritical mass.1

This method relies on the rapid assembly of these components to ensure that the material reaches critical mass quickly, preventing the heat from the initial reaction from dispersing the fuel before it can be fully consumed. Because the speed of this assembly process is relatively slow, it is unsuitable for plutonium, which has a high rate of spontaneous neutron emission that could cause the weapon to "pre-initiate." Consequently, gun-assembly devices are designed using highly enriched uranium-235.12

In the case of the Little Boy bomb, the design also incorporated a tungsten carbide tamper. This component served as a neutron reflector to increase the number of critical masses in the core and acted as a dense physical barrier to hold the core together upon impact, thereby increasing the overall explosive yield.2