Even if the rifle is incapable of firing (something I wouldn't expect the TSA or the CSA to verify), it is still a rifle and must be treated according to policy:
http://southwest.com/travel_center/guns.html
Firearms must be encased in a hard-sided, LOCKED container that is of sufficient strength to withstand normal handling, as follows:
- A firearm in a hard-sided, locked container may be placed inside a soft-sided, unlocked suitcase.
- A firearm placed inside a hard-sided, locked suitcase does not have to be encased in a container manufactured for the transportation of firearms.
- Only the Customer checking the luggage should retain the key or combination to the lock. No exceptions will be made.