Ah yes, I get the two of them mixed up quite a bit.
About the gun-design nuke, I think the main wad of uranium also has beryllium or something like that in it. Right smack in the middle. When the pellet is shot out of the gun, it hits the main ball, and somehow lets the beryllium (or whatever) do its thing and emit neutrons. Gives the reaction a good kick in the pants, and since you now have a critical mass... let the fireworks begin.