Black Friday isn't just when shoppers rush to stores for holiday sales. It's also one of the busiest days of the year for gun purchases.

In the U.S., there are nine guns for every 10 people. Someone is killed with a firearm every 16 minutes. And every minute, gun shops make about 40 new requests for criminal background checks on people wanting weapons.

On Black Friday, the rush accelerates to nearly two checks a second, testing the limits of the National Instant Criminal Background Check System.

"We have a perfect storm coming," says Kimberly Del Greco, a manager in the FBI division that helps run the system, known as NICS.

Much of the responsibility for preventing criminals and the mentally ill from buying guns is shouldered by about 500 men and women who run the system from inside the FBI's criminal justice center, a grey office building with concrete walls and mirrored windows just outside Bridgeport, West Virginia.

Granted a rare glimpse into the inner workings of the NICS, The Associated Press was able to see first-hand why 512 gun sales a day effectively beat the system last year.

By federal law, NICS researchers must race against the clock: They have until the end of the third business day following an attempted firearm purchase to determine whether or not a buyer is eligible. After that, buyers can legally get their guns, whether or not the check was completed.

This clock ran out more than 186,000 times last year.

The problem is the data.

States voluntarily submit records, which are often missing information about mental health rulings or criminal convictions, and aren't always rapidly updated to reflect restraining orders or other urgent reasons to deny a sale. It's a particular problem on Black Friday, when so many background checks are done at once.

There are more than 48,000 gun retailers in the U.S., from Wal-Mart stores to local pawn shops. Store clerks can use the FBI's online E-Check System, which federal officials say is more efficient. But nearly half the checks are phoned in. Three call centers — in Kentucky, Texas, and Wheeling, W.Va. — take these calls from 8 a.m. to 1 a.m. every day but Christmas.

NICS did about 58,000 checks on a typical day last year. That surged to 145,000 on Black Friday 2013. They're bringing in 100 more workers than usual for the post-Thanksgiving rush this year.

The call centers have no access to privileged information about buyers' backgrounds, and make no decisions. They just type in their name, address, birthdate, Social Security Number and other information into the system. On Black Fridays, the work can be grueling: One woman took a call that lasted four hours when a dealer phoned in the maximum 99 checks.

"Rules had to be stretched," recalled Sam Demarco, her supervisor. "We can't transfer calls. Someone had to sit in her seat for her while she went to the bathroom."

In the years since these background checks were required, about 71 percent have found no red flags and produced instant approvals.

But ten factors can disqualify gun purchasers: a felony conviction, an arrest warrant, a documented drug problem or mental illness, undocumented immigration status, a dishonorable military discharge, a renunciation of U.S. citizenship, a restraining order, a history of domestic violence, or an indictment for any crime punishable by longer than one year of prison time.

Any sign that one of these factors could be in a buyer's background produces a red-flag. FBI researchers then investigate, scouring state records in the federal database and calling state and local authorities for more information.

"It takes a lot of effort ... for an examiner to go out and look at court reports, look at judges' documents, try to find a final disposition so we can get back to a gun dealer on whether they can sell that gun or not," Del Greco says. "And we don't always get back to them."

The researchers must use their skill and judgment, striking a balance between the rights of gun owners and the need to keep would-be killers from getting firearms.

Researcher Valerie Sargo says outstanding warrants often come up when they examine a red flag, and that can help police make arrests.

"It makes you feel good that this person is not supposed to have a firearm and you kept it out of their hands," she says.

It also weighs on them when red flags aren't resolved within three days, which happens about two percent of the time, or 512 checks each day on average. Tacked to a cubicle wall, a sign reads: "Our policy is to ALWAYS blame the computer."

These workers have considerable responsibility, but little independent authority.

"They won't proceed or deny a transaction unless they are ABSOLUTELY certain the information they have is correct and sufficient to sustain that decision," FBI spokesman Stephen G. Fischer told the AP.

FBI contractors and employees oversaw more than 9 million checks in the first full year after the system was established as part of Brady Handgun Violence Prevention Act in 1998. By last year, they oversaw more than 21 million. In all, only 1.25 percent of attempted purchases are denied. Denials can be appealed.

People can get guns without background checks in many states by buying weapons at gun shows or from individuals, a loophole the National Rifle Association does not want closed. But even the NRA agrees that the NICS system needs better data.

"Any database is only going to function as well as the information contained within," NRA spokesman Andrew Arulanandam says.

Del Greco doesn't see the states' data improving soon, which only adds to the immense challenge of getting through huge numbers of requisite checks on Black Friday.

"It's really critical that we have accurate information," Del Greco says. "Sometimes we just don't."