One Friday night last month, a man at Buffalo Niagara International Airport was caught with a loaded .38 in his carry-on at a security checkpoint. The next day, the scene repeated itself at Reagan National, where a woman was found carrying an unloaded .22-caliber handgun and 23 rounds of ammunition. And that Sunday, TSA officers at Dulles International detained a man for bringing an unloaded .45-caliber revolver through security.

And those are just the incidents that made the news. A check on the TSA’s official blog shows that during the most recent week of reporting, June 5–11, TSA officials found 56 guns in passenger carry-ons at airports across the country, from Fort Lauderdale to Juneau. Forty-five of those guns were loaded; 18 actually had a round in the chamber.

It’s a trend that some Americans may find surprising in light of the post-9/11 security crackdown: More and more people are trying to board planes with guns on their person or in their carry-ons. Last year, 2,212 guns were intercepted in carry-on bags at airport checkpoints (83 percent were loaded). That’s a 50 percent increase from 2010, when 1,123 guns were discovered.

The reason this is happening? Part of it is just human nature: We’re forgetful. “Particularly if [people] are carrying something light, like a little .22, it becomes like their keys or their cellphone, and they just forget they have it on them,” Jeffrey C. Price, an aviation expert, told the New York Times. A columnist at AmmoLand, who estimated that one out of every 10,000 concealed-carry permit holders inadvertently totes a firearm in their carry-on, put it another way: “I know that you are conscientious. I know that I am conscientious. But are you conscientious 10,000 times in a row?”

But the bigger culprit behind the uptick in guns at security checkpoints is the growing ubiquity of concealed-carry permits. According to the Crime Prevention Research Center, some 12 million Americans had concealed-carry permits last year, compared with 8 million in 2011. Extend the timeline, and the surge becomes even more dramatic: From 1999–2007, the overall number of new permits increased by about 240,000 per year. From 2008–2011, that figure tripled, to 844,000 new permits per year. And from the end of 2011–2014, a whopping 1.4 million new permits were being issued each year. (The center also notes that its numbers don’t account for every citizen who can legally carry a gun, because five states and much of Montana do not even require a permit.)

Further muddying the water: It’s still legal to carry guns as far as security checkpoints in airports in 44 states, including Dallas/Fort Worth International — incidentally, the U.S. airport that saw the greatest number of guns in carry-ons last year, at 120. “I don’t really like the idea of people coming here with weapons and carrying them into terminals,” Alan Black, public safety chief at Dallas/Fort Worth, told USA Today, “but that’s their right as citizens of the state of Texas.”

Black was speaking in 2008, as the TSA was weighing a first-of-its-kind request from Hartsfield-Jackson Atlanta International Airport (which ranks just below Dallas/Fort Worth in the number of guns in carry-ons) to ban firearms in both unsecured and secured areas. A spokesman for Hartford-Jackson said at the time that someone firing a gun there would force a massive evacuation that could disrupt flights at the country’s approximately 450 commercial airports.

However, after the ban was put in place, the gun-rights group successfully pushed for a change in state law to counter it, and Hartsfield-Jackson amended its rules to allow guns in unsecured areas — as amply shown this month, when a Georgia man blithely carried his AR-15 assault rifle and 100 rounds of ammunition through the world’s busiest airport.