Levy's History of the United States Constitution (1986) has an article on the Ninth Amendment. (I gather it ought to be called the ignored amendment.)
Largely ignored throughout most of our history, the Ninth
Amendment has emerged in the past twenty years as a possible
source for the protection of individual rights not specifically
enumerated in the Constitution's text. Although no Supreme
Court decision has yet been based squarely on an interpretation
of the Ninth Amendment, it has been mentioned in several
leading cases in which the Court enlarged the scope of
individual rights. Lawyers, scholars, and judges are
understandably intrigued by a provision that, on the basis of
language, seems ideally suited to provide a constitutional home
for newly found rights: "The enumeration of certain rights in
the Constitution shall not be construed to deny or disparage
others retained by the people."
The historical origins of the Ninth Amendment lay in James
Madison's concern that the inclusion of specified rights in
the Bill of Rights might leave other rights unprotected.
He recognized, moreover, that the inherent limitations of
language could thwart the intent of the authors of the Bill of
Rights to provide a permanent charter of personal freedom.
The 9A "lay dormant throughout most of our history". To summarize: the Supreme Court ruled in Barron v Baltimore 1833 that the Bill of Rights was a limitation on the United States, not on the states. The assumption was state government would protect its people and the only souce of oppression of rights would be the federal government. The states had broad police powers to regulate welfare, health, education, morality andbusiness; SCOTUS review of state legislation was limited to how it affected interstate commerce.
13A and 14A passed after the Civil War were intended to guarantee iindividual rights for former slaves (and everyone else) against state infringement. SCOTUS in the Slaughterhouse Cases 1873 moved to reassert the principle established in Barron v Baltimore 1833 that the Bill of Rights protects individual rights from the federal government, not the states. Extention of protection of individual rights against oppression by the states by 13A and 14A were very narrowly interpreted by SCOTUS. Essentially post-Civil War SCOTUS enabled the rise of Jim Crow with Slaughterhouse 1873, Cruikshank 1876 and the Civil Rights Cases 1883.
The 1930s and 1940s began to see interpretation of the 14A to incorporate enumerated rights of the Bill of Rights through the due process of clause of the 14A and by the end of the 1960s the 1A, 4A, 5A, 6A and 8A were made binding on the states.
In this legal development the Ninth Amendment
was inconsequential, because the Court employed the judicial
technique of incorporating into the due process clause [of the 14A]
rights enumerated in the Bill of Rights. There was little need to
develop the concept of "unenumerated" rights, so long as the
due process clause of the Fourteenth Amendment provided the
vehicle for making the Bill of Rights binding on the states.
9A was cited maybe in one major SCOTUS case. In Griswold v Connecticut 1965, Scotus struck down a Connecticut law that outlawed contraceptives; the goal of the state with that law was to discourage sex outside marriage by banning birth control to everyone (Griswold's crime was discussing contraception with a married couple). Justice Douglas' opinion was to find an unenumerated right to marital privacy. Three of the nine cited the 9A as protecting unenumerated rights. The majority preferred to find the unenumerated right within "penumbras, formed by emanations" from enumerated rights (1A, 3A, 4A, 5A) rather than rely on the untested 9A.
I would argue that the individual right to keep and bear arms is
specifically enumerated and protected in the operant clause of the 2A ("the right of the people to keep and bear arms shall not be infringed") from the beginning, and that the miitia clause ("A well regulated militia being necessary to the security of a free state") is an example not a limitation of reasons to protect that right.
First, Heller and McDonald are based on the interpretation of the militia clause as an 18th century example of citing a reason important to the government to not infringe "the right of the people to keep and bear arms". In times of national emergency, how do you raise a voluntary force (militia) from a people with no familiarity with use of arms? But the militia example is a reason. not the only reason, to preserve RKBA.
Second, to shift protection of individual RKBA from the 2A itself to the untested 9A would be abandoning court precedent on 2A in Heller '08 and McDonald '10 for a relatively untested interpretation of 9A.