The Annoyed Man wrote: That's the thing.... a lot of pellet guns look like a real firearm, even fairly up close. And even if you were to take the time to squint at the muzzle to try and see what it was, under duress how many of us would be able to tell the difference at a quick glance between a .22" aperture and a .177" aperture? I'm not waiting to find out. I'm not even waiting to see what comes out if he shoots first. I'm not going to give him the opportunity to shoot first.
Try and convince the guys at the airport TSA checkpoint that it's 'just a pellet gun'. Even with the ability to calmly assess it and determine its true nature, you're still not getting it on the plane with you.