I don't quite understand the argument that gun ownership is a fundamental natural right. Certainly there haven't always been guns so it wouldn't make sense to call that specifically a right.
Maybe it derives from a fundamental right to property. But if that is the right you are trying to defend, I'm afraid that bird has already flown. Governments restrict our permission to own numerous substances. You can't carry around explosives, uranium, or marijuana amongst many other things. We don't even let you buy or sell your own organs.
Of the numerous property rights infringed upon or proposed to be infringed upon by the state, why exactly does the prospect of "gun ownership" being slightly restricted cause so much ire?
Or perhaps the argument is that it derives from the right to defend oneself. However, a restriction on gun ownership certainly doesn't prevent you from 'defending yourself', rather it simply restricts the tools and circumstances under which you can do so.
We wouldn't want to say you can defend yourself through whatever means you want, as that would lead to a license for private citizens to keep tanks, explosives,or whatever else they can make an argument for that it makes them safer. Your neighbor could line his front yard with a mine field.
The right to defend oneself when construed very broadly is even a shady natural right on the face of it.
If we were to affirm it completely we would be willing to say it is OK to go over to your neighbor's house and shoot him in the head merely because that neighbor is considering doing you some harm.
Surely the right we want to defend must be some sort of qualified right to act within certain reasonable constraints to defend oneself. Constraints that are defined by the norms of the society.
And once you accept some sort of characterization of the principal rights in terms of the society, then small modifications of one's right to bear arms no longer seem like such a flagrant violation of libertarian principles anymore.
And the Constitution has been changed many times. Ask women and blacks.
Changes are simply evolutions of society's judgment on what is or is not an acceptable behavior.
And we surely need to continue evolving, or join the crowd at the bottom of the pool.