Nearly two years after Mayor Bloomberg launched his legal battle against 27 out-of-state gun salesmen, the city is being forced to prove that it targeted the right ones.

Most of the gun dealers, whose businesses stretch between Ohio and Georgia, have agreed to let the city pay to monitor their business practices in return for the dismissal of the lawsuits. A few went out of business shortly after being sued.

Three of the gun dealers, though, are going to trial over the city’s claim that they have disproportionately contributed to arming criminals in New York. The trial of the first, Adventure Outdoors of Smyrna, Ga., is set to begin May 27 in U.S. District Court in Brooklyn before Judge Jack Weinstein. The court papers suggest that Mr. Bloomberg may testify.

City lawyers claim that stores such as Adventure Outdoors, which is nearly 900 miles by road from New York, are a public nuisance to the city because they sell guns to people who quickly resell them to criminals. Adventure Outdoors sold at least 21 guns between the years of 1994 and 2001 that were recovered in New York City, most of which law enforcement officials considered to be linked to crime, the city claims in court papers. One of the guns was used to shoot a man in the face in Manhattan in 1996, the complaint said.

One name likely to be mentioned at the trial is that of a store clerk, Mohammed Ali, who, more than a decade ago, was pistol-whipped in Manhattan with a gun that the city says was sold 16 months earlier by Adventure Outdoors.

All told, the evidence does not support the “idea that Adventure Outdoors is responsible for all the blood in the city,” a lawyer for the gun store, John Renzulli, said.

“They are not able to present one homicide or instance of a police officer being shot with one of our guns,” he said.

The cost of the city’s suits against individual, out-of-state gun stores — a tactic pioneered under Mr. Bloomberg — has run into the millions. An investigative agency, the James Mintz Group, which conducted a sting operation at each of the stores facing suit, has billed more than $2 million, although its original contract was for $800,000, according to court documents.

The court-appointed attorney, Andrew Weissmann, who now monitors the gun stores that have settled lawsuits with the city, is working for free. But two associates at Mr. Weissmann’s firm, Jenner & Block, bill the city at $250 an hour for their work, a city official who asked not to be identified said. Other expenses, including hiring a consultant as well as travel to the stores, has reached $180,000, the official said.

The city’s criminal justice coordinator, John Feinblatt, said in a statement that the lawsuits are already producing results.

“Crime guns recovered in the City from the five states where dealers were sued dropped by 16%” last year, Mr. Feinblatt said in a statement, sent via e-mail. The number of crime guns from some other states where no dealers were sued was up, he claimed.

One potential difficulty in the city’s case relates to the period between the sale of a gun at Adventure Outdoors and its recovery in New York — a lag known to gun investigators as the “time-to-crime.”

“To point your finger at the original seller of the firearm is the same as trying to make a car dealer responsible for where a car goes or if someone speeds with it,” the owner of Adventure Outdoors, Jay Wallace, said.

City lawyers claim that the guns it linked to Adventure Outdoors have an average time-to-crime of three-and-a-half years.

Because guns can be resold legally after their purchase from a store, the Bureau of Alcohol, Tobacco, Firearms and Explosives doesn’t generally consider a time-to-crime of that length to be a sign that a specific dealer is skirting sales regulations.

“We’ve found that with anything longer than two years the gun has passed through too many hands,” a spokesman for the New York office of the ATF, Joseph Green, said. “We’ve found that a two year time-to-crime is a strong indicator of potentially illegal firearms trafficking,” Mr. Green said, emphasizing that he was speaking generally and not about the city’s suits.

Mr. Wallace described the 21 guns that the city had linked to his store as “an amazingly small percentage” of the overall number of guns that he has sold in his more than 30 years in the business.

“We’re very cautious when selling firearms,” Mr. Wallace, who is suing city officials for defamation in connection to the suit, said. “I’m confident about the way I run my store.”