I would point you to the FAQ but some genius mod/admin deleted a whole bunch of the threads there, including the "How to fly with your airsoft thread".....<sigh>
Anyway, I've been flying with airsoft items for over a decade, here is what you do;
1. Bring rifle to the airport in a gun case, unloaded, no battery in it; gun case must be locked.
2. Inform the person at the check in counter that you are travelling with a firearm and request to fill out a "firearms declaration form" (it's yellow).
3. Fill out the form.
4. Keep your copy.
5. Bring your rifle to oversized baggage.
6. The dude there will scan it, you may be asked to open the case ( i was one time, turns out the scanner fellow was from bosnia and the AKM I was transporting caught his eye, two RCMP officers showed up asked me to take it out of the case. Everyone took turns checking it out with "cool" and "neat" being the comments of the day, and I put it back in my case and it was on it's way to the airplane.
7. Enjoy your flight!
That is the quick and dirty, 100% legit guide to flying with airsoft. If you have questions feel free to PM me here.