Get a very good quality rigid case and check it like a regular gun. Even tell them it is a XX caliber gun. Leave out the whole "air rifle" spiel. Keep it simple, the TSA is easily confused and alarmed. Physics is a funny thing. At altitude, the pressure outside of the air tube is 5 PSI, instead of 14.7 PSI, a difference of roughly ten PSI. Or the equivalent of having the air tube at 3010 PSI at sea level, as versus 3000 PSI (differential pressure is all that matters). The bigger risk is seal failure at the extreme cold temperatures found in the belly of the plane at altitude.