You’re correct that the 2nd Amendment (and the Bill of Rights as a whole) did not apply against the states when they were originally adopted. But, let’s see, did anything happen since Barron v. Baltimore was decided in 1833? Oh, the Civil War, that’s right. And after the Civil War Congress adopted (wait for it) The Civil War Amendments, including the 14th Amendment, an amendment whose author, Congressman John Bingham (R-OH), specifically declared was meant to incorporate against the states the Bill of Rights (as well as the fundamental rights enunciated by Justice Bushrod Washington in Corfield v. Coryell). While the Supreme Court initially refused to incorporate the Bill of Rights against the states (in The Slaughterhouse Cases, 1873, which basically read the 14th Amendment’s Privileges or Immunities Clause out of the Constitution), it began the process of “selective incorporation” against the states of almost all of the clauses in the bill of Rights commencing in 1897 but not picking up speed until the 1960s. Judge Sotomayor’s citation of pre-1897 cases for the proposition that the 2nd Amendment should not apply against the states is risible, given that *of course* the Supreme Court held back then that the 2nd Amendment only applied against the federal government, since back then none of the Bill of Rights had been incorporated against the states.
Non sequitur.
the 14th Amendment, an amendment whose author, Congressman John Bingham (R-OH), specifically declared was meant to incorporate against the states the Bill of Rights
And as Charles Fairman observed, what "Bingham said in 1871 formed no part whatsoever of the facts that produced the Fourteenth Amendment...He had made history, but his afterthought should not be allowed to remake it."