Much as I like guns, I think guns and planes don't mix. Especially ones that carry lots of people.
Great shot as I am, I'm just as likely to hit someone else or puncture the air seal on a plane as I am to hit the perp.
I'd sooner just wait for a hijackers to walk by, then pounce on him and twist his head off.
I think this is a moot point. Since the events of 9/11/2001, the ability of Americans to be cowed by hijackers on planes is much reduced. I think any time they try it again, they will be pounced on by everyone, and would be lucky to survive long enough to be take into custody.