Thinking of it as "guns" being a fundamental right is... kinda backwards.
And, for the record, I think that "because the Second Amendment" is a horrible and fantastically lazy argument for gun ownership.
Now, let us take a look at the Universal Declaration of Human Rights. Specifically, Article 3:
"Everyone has the right to life, liberty and security of person."
From this, I don't think it's a stretch to say that everyone has the right to protect the above -- within reason!
Everyone has an innate right to defend their life, liberty, and security of person. Without that, those rights are meaningless.
This is not the job of the police. You should definitely call them if you have the time! But they are under no obligation to protect you, or even to show up.
(Most people don't know that, by the way.)
When you encounter a threat to the above, you have the right to fight until that threat ends, with some leeway granted for the adrenaline dump of combat.
This is not an indiscriminate license to kill or even to engage in violence!
Even if you "feel threatened". You must be able to demonstrate that you acted in defense of life -- yours or another's -- in order to justify your use of force against an attacker.
Weapons are the means by which individuals use force.
Allowing for self-defense allows for the use of weapons.
There is no such thing as a "non-lethal" weapon.
Your hands and feet are natural, and lethal, weapons. More people die by these every year in the United States than by "assault weapons".
Clubs and knives? Definitely lethal. The difference between the winner and a loser in a knife fight, is that the winner dies on the way to the hospital.
Tasers kill more people annually in the United States than "assault weapons". Tasers are also one-time use only. Miss your attacker? Too bad. Attacker has a thick jacket? Better luck next time. Multiple attackers? That sucks.
Pepper spray is probably the least-lethal thing you can use, but it still can and does kill people. Pepper spray is horribly ineffective. Use it indoors, or against the wind, and you will incapacitate yourself. It is useless in the rain. Or if your attacker is wearing a face mask. Or if you have multiple attackers.
A gun, on the other hand, works in the rain. It works in the wind. It works equally well regardless of how strong or weak you are. It works against multiple attackers.
Guns, even if "silenced", are also amazingly loud, which is a big advantage when you're defending yourself, because you want people to hear that noise and call the cops.
So, in a world where we respect peoples' human rights, allowing gun ownership really is common sense.