BRIDGEPORT, W.Va. — 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 gray office building with concrete walls and mirrored windows just outside Bridgeport, West Virginia.