There is no logistical reason that a single person could not own, maintain, and deploy a nuclear device that was assembled for them (like we buy guns that were assembled for us by a manufacturer).
A nuke is only different because it is more rare, but the Davy Crockett nuclear recoilless rifle could be mounted in the back of a pick up truck.
Wrong. Very, very wrong.
Though you are correct about the rarity of nuclear devices, your statement shows a complete lack of knowledge of what it takes to both initiate a successful nuclear yield from a nuclear weapon and what it takes to keep just the warhead alone servicable over time. For even a "simple" fission warhead, there are nuclear and physical aspects about the Plutonium which have to be met in order to undergo an explosive nuclear release, one of which is the Plutonium must be in the Delta phase (unless a specific design uses another phases)...one of 7 possible allotropes of the element. Pu can undergo phase transition from one phase to another just due to variations in temparature, pressure, and chemistry. It's also a very chemically reactive element. Even if the Pu is stabilized by alloying it, it can still very easily undergo phase transition into the Alpha phase, which would render it useless.
Things get radically more complicated if it's a boosted fission warhead and an order of magnitude more complicated yet if it's a thermonuclear warhead.
People seem to think that producing a nuclear yield from a warhead is something that's simple...just bring two pieces of U-2325 or Pu-239 together and you get a nuclear explosion. Hardly. In fact, one of the best security angles with respect to nuclear weapons is that they're so easily rendered inert as a nuclear device by simply upsetting the delicate balance of either the materials or the sequence of events which are required to initiate a nuclear yield. Just ask N. Korea how difficult it is, and they're TRYING...their third test still hasn't actually been confirmed as having a real nuclear yield and at any rate is significantly less than 10Kt.
Though U-235 can be used to produced a much simpler nuclear device, it still has it's own difficulties. You cited the Davy Crockett...yet if you upset the delicate balance required for its operation, it would also end up being a very expensive door stop.
Most of us can't manufacture and AR-15 from scratch either, but we can buy them.
If it was legal, a person could just as easily buy and truck around a Davy Crockett nuke. Is this a good idea? ... probably not.
Again, there's more than just legality with this. Weapons grade fissile material is extremely expensive and time consuming to make, as is all the processing required to produce a usable warhead from it once you have it. You're talking about a level of undertaking that requires a national effort.
Additionally, there are other concerns with this than just making the fissile material...that of controlling it. And that's a huge concern to the nations that produce them because loss of control of fissile material means many things, not the least of which is providing a rogue nation or terrorist organization with large quantities of refined materials with which they can short circuit a conventional nuclear weapons program. You also have the fact that these are both radioactive elements and, in the case of Pu, extremely toxic. Combine these things and what you have is something that's prohibited to the public on many levels, not the least of which is the possiblity of producing a fission weapon. Environmental and population safety concerns due solely to the radiological and chemical properties of these elements is a huge factor.
Is it a bit hyperbolic? Sure. But the point is that at SOME point in the spectrum of "arms" it has been generally agreed that there is a barrier/line that civilians don't get to cross.
Nuclear bombs are at one far end of the spectrum. Sticks are at the other. Bolt action rifles, RPGs, revolvers, SMAWs, shotguns, tanks, full automatic weapons, artillery, AR-15, etc. all fall somewhere in between.
It's more than a bit hyperbolic...it's so far outside the realm of reasonableness that it's assinine to even consider it as a possibility. In fact, depending on the argument, it can even be considered a strawman argument.
Right now, in the US, the barrier/line is being defined approximately:
1. Semi autos for all, but sometimes with extra paperwork depending on locality
2. Full autos with restrictions and extra paperwork
3. Explosive devices
The barrier/line is never going to disappear entirely, which would be the least restrictive interpretation of "Shall not be infringed" that some advocate for. The question is where that barrier/line will be set, which will be determined through legislation and the courts.
I absolutely concur with you on this.