But a principled approach that relies completely on the Constitution is not very intellectually satisfying either. Libertarians or anyone else that relies completely on constitutionality has to accept the vast majority of the actions of government, since the Constitution grants the government the authority to determine the constitutionality of its own laws and actions. If the Supreme Court says that the Second Amendment doesn't apply to assault weapons, then that is constitutional, at least until the legislature changes the laws, at which point the Supreme Court can again interpret it to mean something else, ad infinitum (or until tricks like judicial stripping get used). They have to accept everything that is or ever was constitutional, like slavery, prohibition, searches and seizures when there is a warrant issued for any reason, direct election of senators, etc. Many "strict constitutionalists" avoid this by appealing to the intent of the forefathers, but then your entire "principled approach" is based on highly subjective interpretations, usually of extra-constitutional writings by various forefathers.