LOL - a shotgun nuke is a sub-critical mass nuke, where the bit needed to make it bigger than critical mass is blown in by explosives. Effectively, you have a mass of uranium that is too small to go off, and you shoot enough extra at it with a shotgun, to make it go off.
Hence, shotgun nuke.
That is how the did it in WWII. And the yeild was entirely dependent on the mass of stuff involved.
More commonly today, they just tickle it in the right place, and interesting things happen.
You mean "gun-assembled." The only gun assembled device ever used was Little Boy on Hiroshima. Fat Man was an implosion type device and the trinity device was also implosion.
Our first use was Gun assembled because the men who designed it knew it would work and thus wanted to make sure the first "demonstration" worked. Nagasaki was to make sure the Trinity type device worked in a operational environment and since Hiroshima was already leveled a fizzle would not have been quite as embarrassing.