The Billings Airport does have, and has for a number of years had, regulations concerning the possession of firearms in any of the Airport's facilities, including the Terminal Building.
The current Airport regulations state:
"No person shall carry or possess a firearm or destructive device on the Airport, except:
(a) Firearms enclosed in a carrying case or other container for shipment by air; and
(b) Firearms carried by peace officers, government employees, or members of the Armed Forces of the U.S., when such person is on official duty which authorized the possession of a firearm."
Remember that the airlines and TSA also have regulations regarding the carrying and transport of firearms and weapons. Be sure to check with both the TSA and the airline on which you are traveling to review their regulations on firearms and weapons.