A “gun shop” that suddenly appeared in the heart of Manhattan last week only to close its doors after two days was actually an elaborate ruse by a pro-gun control group that sought to lure potential customers in order to make their anti-firearms case. States United to Prevent Gun Violence set up the “Guns with History Gun Shop” on Manhattan’s Lower East Side, and pretended to offer for sale guns used in high-profile crimes. The phony sales pitches were really part of a “social experiment”—pretexts for telling customers about the dangers of gun proliferation in unconsummated transactions recorded by hidden...