The point of inalienable rights, is the realization that you are born with certain inherit abilities granted upon birth. You can speak, walk, and rationalize and no government can determine your capability to do these things, therefore the government has no authority limiting these things.

Likewise, you have the right to defend yourself, family, property, and possessions from harm and theft. Therefore you have the right to arm yourself in order to ensure you can adequately do this. You also need the ability to defend yourself if, as in the 13 Colonies example, your government infringes your safety or violates any of your inalienable rights such as your speech or self defence.

So the right to a weapon does not violate your idea of “natural law” but instead ensures your ability to function capably within the confines of natural law.

A Constitution does not “grant” rights you already have, but instead forces your government to recognize those rights and respect them.