All good info regarding actual firearms. I know it is not overly difficult to travel by air with them.
But my OP was about traveling without a firearm, but with what panty wetters might think is a firearm.
I will not sign a declaration that I have an unloaded firearm when traveling with a replica.
First, I don't trust that (even an airlines printed) form has not been "federalized" by dint of being interstate air travel. It could be a perjury trap. A vindictive US Attorney could charge one with claiming a firearm when there was no firearm.
Second, if flying to a NYC destination (not connecting there) having declared a firearm(handgun), I could be arrested as I am not permitted to have a handgun there. When they see that it is not a firearm, the accusation would be that I pulled a "switch" and that I was a gun runner.
Third, upon return to Massachusetts with a "firearm" it is not impossible that our AG could chase me to FA-10 the non-firearm. There would be a paper trail of me admitting that I was traveling with an unloaded firearm.
Sure -- none of that would happen with reasonable people. Trusting that all I'd come in contact would be reasonable, is not something I'd like to risk.
I think the best approach would to tell the ticketing agent that I have a replica, that it is not a firearm. I would insist on not treating it like a real firearm. I fear that this might cause some delay, and I think I'd have to be prepared for that.
[This question was raised because a TSA fact sheet explicitly says that replicas cannot be taken on carry-on baggage (obviously, and no complaint), but that they are admissible in checked baggage. It just didn't say what the checked baggage proceedure was.]