2. How many people can a weapon kill in a short time before gun rights advocates deem it excessive?
That's the real question here. Nukes are merely an extreme. Whatever number the gun rights advocates decide upon, the human safety advocates will decide upon a lower one.
Besides, I thought it was all about the responsibility of the person owning the weapon. If the person is responsible, licensed and trained, then what difference does it make whether the weapon can kill one person or one million?