Over the past few years, Blogger Bob at the TSA's web site has reported quite a few antique firearms being seized because they weren't properly declared.
I'm probably stating the obvious here, but regardless of what the official TSA definition of a firearm is, at the point where you and the TSA connect, it's up to the agent on the spot to make a determination. I don't travel by air much anymore, but I don't think that I'm going out on a limb by guessing that the agent is going to make the most conservative decision possible, which would be that if it looks like a firearm to him, it must be a firearm. You'll get the opportunity, at some point, to make your case, but it's going to take some time and trouble.
Everyone's probably seen the posts about debates with USPS over what can and can't be shipped, and those regulations are crystal clear, but many post offices still don't have a handle on them. The TSA, for better or worse, seems to prefer to be shrouded in secrecy.
I'd go for the locked case regardless. Some TSA agent is going to paw through your stuff and even if the rules allow a BP revolver through without declaration, it's a relatively expensive doo-dad that I would just as soon see arrive along with my luggage. If it's declared, you can be darn sure that it will turn up on time.
Well we don't rent pigs and I figure it's better to say it right out front because a man that does like to rent pigs is... he's hard to stop - Gus McCrae