Exactly what law prohibits me from possessing a functional nuclear weapon in the USA?

I found that magazine in a local library here in Australia in the late 80’s. Have a feeling it was Popular Mechanics, or something very like it.

I assume you’re just joking, but just to clarify the Federal Govt can and did impose a federal restriction on all fully automatic weapons in the 1930s without modifying the Constitution in any way…The NRA nor any other so-called ‘gun-nut’ group has ever seriously challenged them. BTW, fully automatic ‘machine guns’ are not totally banned in the US like heroin or something, you can get a federal permit for one. You just have to jump thru a whole lot of red tape.

Yes, this is a big misconception. Although making fissionable Uranium or Plutonium is incredibly difficult & expensive, the rest of the bomb is almost as complex & costly to manufacture as well. Implosion is an order of magnitude harder than the ‘gun-type’. So much so that South Africa, which has copious deposits of Uranium ore, chose to just make a dozen or so gun-type weapons in the 70s (thank god they destroyed it all after the, ahem, natives took over. :D) Also look at North Korea. They worked furiously on building one for decades and the best they could come up with was a couple of essentially fizzling, smaller than Little Boy duds.

A thermonuclear (i.e. H-Bomb) is another order of magnitude further in complexity (again, thank god!)

But not by much. The Little Boy design is quite well known at this point. For instance, it’s known that the two subcritical masses were a solid and hollow cylinder (which nested), and that the hollow cylinder was the moving piece. The design wasn’t even tested before it was dropped on Hiroshima (Trinity was an implosion design, similar to Fat Man).

North Korea’s efforts could have fizzled for all kinds of reasons beyond the basic weapon design. Getting pure U-235 really is the hard part when it comes to gun designs.