Where's it say so in the Second Amendment?
Doesn't have to. It "says so" in the regulations.
A regulation is a rule or order having force of law issued by executive authority or government.
Regulations are issued by various governmental departments to carry out the intent of the law. Agency issue regulations to guide the activity of those regulated by the agency and their own employees and to ensure uniform application of the law. Regulations are not the work of the legislature and do not,
in theory, have the effect of law.
In practice, however, because of the intricacies of judicial review of administrative action, regulations can have an important effect in determining the outcome of cases involving regulatory activities.
My underlines, rest is directly from Blacks Law Dictionary
As I read it, when the "law" (i.e. 2nd Amendment, etc.) seems to be ambiguous, unclear or contentious, the courts will fall back on the "regulations" for guidance. As case law is made and the "regulations" are upheld in appeal, they become more and more the same as "law".