The was no word whether that includes the passenger being arrested.
Delta Airlines said the passenger boarded a flight headed for Narita International Airport in Tokyo.
The passenger later discovered the firearm and alerted authorities, who met the plane when it landed on January 3 in Tokyo, officials said.
The screening took place amid the ongoing partial federal government shutdown during which TSA workers aren't being paid.
The federal agency, according to statistics posted on its website, had 7.6 percent unscheduled absence rate on Monday, compared to a 3.2 percent rate on the same date a year ago.
TSA Administrator David Pekoske last week announced that the agency would pay its employees for work conducted on that first day of the shutdown and awarded bonuses of $500 to each of them.
The closure did not appear to cause a surge in long lines on Saturday, and passengers were mostly sympathetic to the plight of the unpaid workers.
More than 290 weapons had been discovered in 2018 through the beginning of November in Atlanta, the busiest airport in the country, according to the Justice Department.
Trump is considering a possible national emergency declaration that would end the shutdown and allow him to obtain his wall funding by circumventing Congress.
About 51,000 TSA agents are among the 800,000 government employees working without pay or on furlough during the shutdown.
As the government shutdown continues into a third week, TSA staff have been calling out sick in record numbers.
"Passengers are encouraged to arrive early and give themselves extra time to check in for their flights and to clear the security checkpoints", the airport said.
TSA said that it screened 1.97 million people on Sunday and that 99.1 percent waited less than 30 minutes, and 93.1 percent less than 15 minutes.
"Security standards remain uncompromised at our nation's airports", TSA spokesman Jim Gregory wrote in an email.
Meanwhile at Fort Lauderdale-Hollywood International Airport, all checkpoints were open this weekend and appeared to be operating normally.
"We look at our TSA partners as a part of our TPA family and hope to help them during this time", the airport tweeted on Friday, using its three-letter code.