A federal appeals court threw out New York City’s longstanding lawsuit against the gun industry on Wednesday, ruling that a relatively new federal law protects gun makers against such suits. The appellate ruling killed perhaps the boldest avenue by which the city has sought to stem the flow of illegal guns into New York: a claim that gun makers and distributors have knowingly flooded illicit, underground markets with their weapons. The city’s suit, filed in 2000, was upheld in December 2005 by Judge Jack B. Weinstein of Federal District Court in Brooklyn. Judge Weinstein allowed it to move forward, despite...