They tend to lean on whether it is reasonable that the Founders might have had access to such a weapon with their technology. Machine gun is just a rifle with automatic rechamber. Not an unreasonable upgrade for 1700s technology. Maybe, I dunno; political people don't have to actually care about the details.
There are limits. And if cases like this made it there they might rule that no Founder was smelting the materials. That they would have had to collaborate, in some "market dictates options" ruling to limit hermits going in a rampage. Also everyone a weapons assembly line in their home is anti-corporate capitalism.
"George Washington understood the value of civic life and sound economics! He would not have tolerated such insular selfishness! He did not make his own weapons! He engaged in trade!"
Not saying it's realistic but politics is not never controlled by people living in reality. Making shit up seems as reasonable as anything.
Basically everything else can be owned with an NFA tax stamp. Nuclear weapons my understanding is the difficulty is more with laws on handling the material than specifically owning one as a weapon, so I'm unsure those are even outright illegal either.
Explosives are actually one of the ones with looser restrictions. Even felons can own and re-instate their explosives rights, because bafflingly when congress de-funded the firearms rights restoration process for felons they forgot to do the one for explosives. Felons can also own and manufacture explosive black powder without scrutiny or paperwork, even ones intended to go in a black powder gun.
There’s a whole community of folks building semi-automatic auto-return triggers that are “technically” semi automatic, but with just a gentle squeeze, fire off another. If you maintain that grip, the return mechanism engages, returning the trigger to firing position, where your pressure causes it to fire again… it’s called a Forced Reset Trigger.
My point overall was government is fine with arbitrary exceptions that would get Stan's dad going all "Oh I'm sorry, I thought this was America."