The weapon was reported by a baggage handler after it fell out on Sunday morning. Officials say they screened the bag, but only for explosives.
Airport police say the owner failed to properly declare and package the gun. It's legal to carry an unloaded firearm in checked luggage as long as the airline is notified.
The passenger was later allowed to fly without further incident, but the gun was turned in to police.
The Associated Press contributed to this report.