[Prime Choice]

The precise shape of the core is rather important, and working with the brittle, dangerous metals that are used for nukes is anything but trivial.

Making the core is straightforward using Uranium suspended in hexaflouride with an electrically-charged collector.

The bullet counterpart to complement the concave sphere is cast in the same die as was used for the collector. Perfect fit. Put 'em together. Supercritical mass...and one really bad day.

[mikegi]

Well, since you seem to have some experience in these matters, here's a link to the full story: US-Russia Team Seizes Uranium at Bulgaria Plant. From the article: 37 lbs, 36% enriched.

Doesn't seem like bomb material to me. Perfect terrorist material, though. Everyone has been brought up to believe that any amount of radiation is lethal, so you blow this stuff in an a standard explosion in the middle of Manhattan and people will be afraid to go there for years.

[Southack]

"The bullet counterpart to complement the concave sphere is cast in the same die as was used for the collector. Perfect fit. Put 'em together. Supercritical mass...and one really bad day."

Put them together either too fast or too slow and you'll just get a fizzle, not a BOOM. Likewise, if the shapes aren't precise, your odds for a fizzle increase, and shaping Uranium into a proper core is non-trivial.

Nor can your gun-type barrel permit the gasses to break away a piece of the detonator bullet/core, or even erode any sizable part of the core, lest that radioactive material reach the intended center of the globe/donut far enough in advance to fizzle your device.

If you've ever paid close attention to a firearm firing on the range, you've probably noticed that some gasses actually leave the barrel *prior* to the bullet exiting your rifle. With an open lead ball, some tiny part of that sort of bullet is actually eroded externally into lead gas that likewise escapes your muzzle-loader, for instance, prior to the bullet itself escaping.

Now picture that same erosion occurring with an open Uranium or Deuterium bullet, sending radioactive gasses into your core prior to the critical mass being assembled.

These are non-trivial issues, and they only scratch the surface of the problems faced by getting a working nuke to become operational and reliable.

Yes, it can be done, and having the fissionable material is a big step toward getting there, but the rest of the equation is still non-trivial is my point.