(CNN)-- Passengers are not allowed to carry guns onto airplanes, yet the Transportation Security Administration finds hundreds of weapons each year.
In 2012 more than 1,500 firearms were discovered by screeners at airport checkpoints, TSA spokesman David Castelveter said. That is an average of more than four weapons a day.
A total of 1,320 firearms were found in 2011. Many were loaded, and some had rounds in the chamber.
"We do assume that the vast majority of weapons that come through are (from) people who didn't know they couldn't carry them on or were not aware they were in the bag," Castelveter said.
Through the end of November, Hartsfield-Jackson Atlanta International Airport led the nation with 80 guns found.
The top five airports also included Dallas/Fort Worth International with a total of 75 firearms, Phoenix Sky Harbor International Airport with 50, George Bush Intercontinental Airport in Houston with 48, and Denver International Airport with 36.
It's not clear whether the increase in the number of guns found in 2012 is due to more people traveling with weapons or TSA screeners and new screening technologies doing a better job of finding them.
When firearms are found at a checkpoint, the TSA calls local police who determine whether the passenger should be arrested, ticketed or have their gun returned.
"They can be permitted to take the weapon back to (their) car and come back," Castelveter said. "If it is illegal (to have the weapon in that locality), then it is confiscated by law enforcement."
Under TSA rules, guns are allowed in checked baggage in locked, hard-sided containers. They must be unloaded and declared to the airline, and passengers must follow local gun laws.
Guns, ammunition, firearm parts and even realistic replicas cannot be carried on the plane.
Guns are not the only weapons banned from the passenger areas of planes. The TSA blog highlights a wide variety of prohibited items discovered in carry-on bags.
In the first week of December, passengers tried to take 40 stun guns through checkpoints. The next week, a walking cane with a sword inside was found, and in the week before Christmas, a spear gun was discovered. In addition, four inert grenades were found inside carry-ons.