Trouble arrives upon their return to La Guardia Airport or Kennedy Airport to fly home. The visitors repeat the procedure practiced at their home airport, presenting the firearm to a gate agent to be checked. Only this time, the gate agent calls police officers from the Port Authority of New York and New Jersey, which oversees the airports. The gun owners are then placed under arrest.

There were 42 arrests for gun possession in the two airports in 2012, according to the Queens district attorney’s office. Some charges of criminal possession of a weapon bring minimum sentences of three and a half years in prison. Like many arrested in New York, the gun owners can wait a day or more behind bars for a hearing before a judge.

“They’re locked up the same as anybody else on any other felony charge,” Mr. Kane said. “People’s lives are really, really disrupted, as you can imagine.” Mr. Kane has represented so many of these tourists that he has posted a greeting to prospective new clients on his Web site.