So by useless you mean absolutely essential?
Hard to argue the fair use thing but it was designed around humans not AI so just because it is fair use now does not mean it should or will be in the future.
I dare say, this thought that people so well read they could architect a country were not aware of the State of the Art in weaponry is rather naive.
Gatling Guns came well after the Founding, were large, heavy, hand cranked, cost between $1000 to $1500 to purchase and required a four man crew to operate. I don’t feel they can be compared to modern weapons.
The Founders were intelligent and capable men but they were not infallible. They knew that too, that’s why Amendments exist.
I can see an argument over nukes, but this "they didn't imagine assault weapons" is just some kind of gaslighting. They imagined sufficient arms to defend oneself against authority.