I believe a sufficiently advanced model could provide a layman with actionable step by step instructions for building a nuclear weapon. They're complicated but not (AFAIK) that complicated. The more or less insurmountable barrier there is weapons grade material. Thankfully refinement is prohibitive in cost, expertise, and equipment.
In comparison, basic munitions are incredibly simple given a recipe and shop tooling. But just because something is conceptually simple doesn't mean it's a good idea to go out of the way to disseminate step by step instructions.
A gun type maybe. But then, two paragraphs and some machining knowledge + shop tooling could do the same, given enough refined material.
Ain’t no way a layman is pulling off an implosion device, regardless of tooling or LLM guidance. The explosive lense structure and timing required is quite complex, and would require some significant calculation from someone who actually knew what they were doing.
Nation state, or even sufficiently motivated big corp, if they had the refined material? Sure. Layman? No.
Thinking they can with LLM slop involved? That will make for some very interesting radiological incidents though!