The stings, described in a city report released Wednesday, were conducted at seven gun shows in Tennessee, Ohio and Nevada. Those states are among the many that permit private unlicensed dealers, known as "occasional sellers," to sell weapons at gun shows without conducting background checks.
Gun-control advocates say the loophole makes it easier for criminals to acquire guns and prevents law enforcement from being able to trace those weapons if they are used in crimes.
Nine states, including New York, have passed laws to close the loophole, requiring background checks on at least all handgun purchases at gun shows. Mayor Michael Bloomberg has long campaigned for Congress to close the loophole, and for states to do it on their own if the federal government does not.
Even in states that haven't closed the loophole, federal law bars "occasional sellers" from selling guns to people they have reason to believe would fail a background check.
This is where the Bloomberg operation says 19 out of 30 sellers broke the law during the investigation, in which undercover buyers wore tiny cameras concealed in baseball hats and purses and audio recorders hidden in wristwatches.
In each purchase, the investigator showed interest in buying a gun, agreed on a price and then indicated that he probably could not pass a background check. Most sellers allowed the purchases anyway, responding in some cases by saying, "I couldn't pass one either," or "I don't care," according to the city's report.
Two assault rifles and 20 semiautomatic handguns were bought this way, the report said.
The 11 dealers who refused sales showed they knew the law.
"Once you say that, I'm kind of obligated not to," said one seller, according to the report. "I think that's what the rules are."
"Fact is, you done told me too much," said another who refused. "I wouldn't sell one to you at all."
The city has no legal authority over the dealers and is using its findings to make a point. A copy of the report is being sent to every member of Congress and the findings will be shared the Bureau of Alcohol, Tobacco, Firearms and Explosives.
"The gun show loophole is a deadly serious problem, and this undercover operation exposes just how pervasive and serious it is," the mayor said in a statement.
The undercover operation took place from about May to August and its $1.5 million cost was paid by city taxpayers. The city hired a team of 40 private investigators from an outside firm to make the purchases.
The sting comes three years after Bloomberg's administration conducted a similar operation focusing on illegal straw purchases at gun shops in Georgia, Ohio, Pennsylvania, South Carolina and Virginia that authorities believe were responsible for selling guns used in crimes in New York City. (A straw purchase is when one person fills out the paperwork and buys the gun for somebody else.)
Bloomberg's administration brought a civil case against 27 gun dealers targeted in its 2006 investigation.
As a result of the suit, 20 dealers are being monitored by a court-appointed special master. One is out of business, two more are expected to be put under monitoring agreements and three were dropped from the suit. A final dealer settled with the city but the terms did not include a monitor.
Investigators in this year's sting also attempted straw purchases at gun shows, and were successful 16 out of 17 times.
The city said it was not planning civil action this time around.