CONTACT TSA! There is a good thread on the subject here somewhere but I could not locate it using search. IF a pcp, TSA must be able to verify there is NO pressure. Ideally the air cylinder needs to be open, as in end of airtube removed ( say on a marauder) for visual inspection. TSA approved case! Locked! Call local ( your airport) TSA ahead of time day of flight They WILL inspect it, you will lock it, they will place a sticker on it. If possible insure it ( not trequired just a good idea. Someone here can likey find the above mentioned post??
Do contact TSA, DO print out all rules/codes and carry them with you and maybe another copy with airgun.