More than 144,000 background checks will need to be performed after the the Friday sales, CNN reported Friday. The three-requests-per-second number is three times the daily average. Some 600 FBI and contract call center employees now must work 17-hour workdays to complete the background checks in three business days, as required by law, FBI spokesman Stephen Fischer said.

"Traditionally, Black Friday is one of our busiest days for transaction volume," Fischer told CNN.

On average, more than 500 gun background checks a day fail because of incomplete information required for a decision, according to the FBI's National Instant Criminal Background Check System, which is responsible for checks on firearm purchases from federally licensed shops.

"We are averaging three checks per second," he said. "The challenge is to have staff keep up with this volume. We do that by limiting personal leave, asking employees to work extra shifts and re-utilizing former … employees to serve in NICS during this busy period."