>into a fissionable target that would achieve critical mass once the bullet whacked into it [...] I can’t imagine the gun method really takes a lot of energy (m

>into a fissionable target that would achieve critical mass once the bullet whacked into it [...] I can’t imagine the gun method really takes a lot of energy (more than a shotgun shell, but not much more?) The gun type is mechanically simple, but impossible to scale up. If you're smacking two subcritical spheres together, each sphere can't be more than 15kg.¹ But wait, Little Boy used 64kg of uranium. How did it do that? "Critical mass" is a function of mass and surface area. A sphere packs the most atoms together, a long skinny rod spreads them out. To increase the amount of uranium used and thus the explosive yield, Little Boy actually fired a 38.5kg cylinder onto a stationary 25.6kg plug. Each half of the pit would have been supercritical, if they were spherical. Gun types also need to assemble quickly, since as soon as any part of the assembly reaches critical mass it will start blowing itself apart. It had 64kg of uranium but 2,300kg of neutron reflecting material and tamper, and the launching charge used 3.6 kilograms of cordite! That would be a hell of a shotgun shell. --- ¹: For pure U-233, which Little Boy didn't actually have, since it was only enriched to 80%. Wartime contingencies and all.