If you can find someone who's legally selling a B-52, you can certainly purchase it. Same with the bombs ($200 tax stamp on each one, though). Fighter jet, same thing. But the cannon has to be dismantled because of the 1986 machine gun ban. Everything else on it is kosher, if you can afford it (missiles are stupid expensive).
The hard part is finding military jets and weapons to purchase. And only a millionaire could reasonably afford to fly such a jet on even an occasional basis.
Don't forget you also need to file the paperwork and notify the ATF when you wish to cross state borders with your flying destructive device.
Crossing a state in a fast aircraft happens quite quickly.
I do recall a couple incidents of private people getting into trouble.
One guy purchased some Russian fighter aircraft, completely disarmed. I think he got into trouble based on some import export restrictions.
I believe another individual got into trouble purchasing an old Soviet nuclear powered submarine. It was also free of any weapons, it was the nuclear nature of the material required to run it that was the issue.
The ultimate extreme example...what is the specific law that bans ownership of a nuclear weapon by a civilian??
The primary laws are not on the nuclear weapon itself but on use and possession of nuclear material. Though there is treaties on the weapons and it really boils down to might equals right on that issue. A sovereign nation is unlikely to allow you to have one in their borders, and people may send a military force to kill you or destroy it or reactors and enrichment facilities involved if you try to make one even in international waters or within the borders of a nation that approves.
You need an army to back you up before you get started. Working in deep fortified underground bunkers not vulnerable to air dropped munitions doesn't hurt.
Then you need a lot of smoke and mirrors cloak and dagger secrecy so you can proceed with tests before you get attacked all while denying a nuclear weapon is your intention. Like say India or Pakistan did, or Iran and North Korea are doing. Once you have a bunch of working ones they tend not to attack you as the cat is out of the bag so to speak, but if they learn you are creating them then you and anyone working around them may end up dead, or kidnapped and tortured for information. Unofficially of course.
Nuclear weapons are not overly complicated themselves. There of course is trade secrets and it may take some tests to figure them out yourself.
Most modern nations could create them quite quickly.
The major limitation is not the technology to assemble them, but the enrichment of the material required. That enrichment process is more difficult.
Once enriched material is possessed the assembly of the device is the easy part.
Some of the older designed nuclear power plants created such enriched product as a byproduct of creating nuclear power.
Convenient by design.
Most modern nuclear power plant designs, or those allowed in 2nd and 3rd world nations, or created with international cooperation and sharing of technology tend to specifically be designed not to enrich material during normal operation into weapons grade material.
There is many different types of nuclear power plant.
That said there is private nuclear power plants, so there is legal ownership of the materials, even in the US. There is also various agencies tasked with keeping control of such things.
Such as the Nuclear Regulatory Commission, United States Department of Energy, as well as international bodies that monitor and track the use of such things.
Too much discussion of such subjects though can upset the wrong people and result in you being on lists that could inconvenience you.
Freedom of Speech and all that.