It has been used to interpret the constitution
In DISTRICT OF COLUMBIA ET AL. v. HELLER the SCOTUS decided that the right to bear arms was an individual right and not a collective right limited to organised militias, and that the Second Amendment to the Constitution merely codified that existing individual right (rather than creating the right).
William Blackstone in Vol. 1, Commentaries on the Laws of England (1765) states:
The ﬁfth and last auxiliary right of the subject, that I shall at
present mention, is that of having arms for their defense, suitable to
their condition and degree, and such as are allowed by law. Which is
also declared by the same statute I W. & M. st.2. c. 2. and is indeed
a public allowance, under due restrictions, of the natural right of
resistance and self-preservation, when the sanctions of society and
laws are found insufﬁcient to restrain the violence of oppression.
SCOTUS refer to this saying:
By the time of the founding, the right to have arms had become
fundamental for English subjects. See Malcolm 122–134. Blackstone,
whose works, we have said, “constituted the preeminent authority on
English law for the founding generation,” Alden v. Maine, 527 U. S.
706, 715 (1999), cited the arms provision of the Bill of Rights as one
of the fundamental rights of Englishmen. See 1 Blackstone 136, 139–140
(1765). His description of it cannot possibly be thought to tie it to
militia or military service. It was, he said, “the natural right of
resistance and selfpreservation,” id., at 139, and “the right of
having and using arms for self-preservation and defence,” id., at 140;
Cite as: 554 U. S. ____ (2008) 21 Opinion of the Court see also 3 id.,
at 2–4 (1768). Other contemporary authorities concurred.
The rationale appears to be that because the right of resistance and self-preservation is a natural right it cannot be a right conditional on military service.
The SCOTUS majority opinion concludes:
Putting all of these textual elements together, we find that they
guarantee the individual right to possess and carry weapons in case of
confrontation. This meaning is strongly confirmed by the historical
background of the Second Amendment.
We look to this because it has always been widely understood that the
Second Amendment, like the First and Fourth Amendments, codified a
pre-existing right. The very text of the Second Amendment implicitly
recognizes the pre-existence of the right and declares only that it
“shall not be infringed.” As we said in United States v. Cruikshank,
92 U. S. 542, 553 (1876), “[t]his is not a right granted by the
Constitution. Neither is it in any manner dependent upon that
instrument for its existence. The Second amendment declares that it
shall not be infringed