This argument is reductio ad absurdem. Every political philosophy is subject to similar complaints if you take a single thesis and extrapolate all of the possible consequences that follow from that thesis if it is unaffected by any countervailing considerations.
There has never been a state that does not provide protection for its members against violence by other members. A state that cannot do that or that refuses to do that is not deserving of loyalty and cannot survive long as an entity. Again, without that guarantee, there is no state, there is anarchy.
Merely that a state has a minimal duty to protect its members against violence does not prevent the existence of countervailing duties and limitations on a state's legitimate use of power to comply with that duty. In the case of libertarianism, a libertarian state can only be maintained and perpetuated when the society is highly homogeneous in culture, morals and ethics, the members are highly educated, and there is a strong ethic of personal responsibility. Without those factors -- and I'm sure there are more -- then libertarianism cannot work because the uneducated and irresponsible sheep will gladly pay someone else to take care of their messy problems like self protection. It comes down to the fact that it's easy to impose a totalitarian regime on people, but incredibly hard to get those people to make the personal sacrifices and take on the personal responsibility necessary to create a libertarian society.
Under your hypothetical, the gun control laws you propose could never happen because the citizens would not permit the sovereign to enact them. If they did get enacted, that would be a sign that the society had abandoned libertarian principles in favor of a more statist regime. No amount of lawmaking or constitutionalizing can prevent people from surrendering their freedom if they no longer believe in it.
Do you believe that this application of the 14th Amendment is what was intended when it was written and ratified?