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Of the six separate opinions the Supreme Court generated last month in Trump v. Barbara, the birthright citizenship case, Justice Brett Kavanaugh’s solo contribution was the quirkiest, and perhaps the most revealing. Aside from its grim implications for the court and potentially for birthright citizenship itself, Kavanaugh’s opinion did another big thing that has not really been discussed. Whether intentionally or not, he may have shown how the court can approve greater restrictions on firearms.

The outcome of Barbara turned on the word “jurisdiction” in the 14th Amendment, ratified in 1868, which provides that “All persons born … in the United States, and subject to the jurisdiction thereof, are citizens of the United States.”

Chief Justice John Roberts’ majority opinion was succinct and direct. Holding that “jurisdiction” plainly means “the power of the United States to govern those within its territory,” Roberts upheld the citizenship of nearly all children born in the U.S.

Justice Clarence’s Thomas’ principal dissent meandered through 91 pages of selective history before agreeing with President Donald Trump’s executive order that only the child of a legally “domiciled” parent qualifies for birthright citizenship. This is despite the fact that “domicile” does not appear in the 14th Amendment or the subsequent statutes.

In his opinion,........

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