Class C explosives
That is essentially what you have. The flares and other signaling devices we carry onboard are basically considered the same as fireworks.
If I was flying with a flare gun in my checked bag I would claim it up front, carry no ammunition (smoke or flare rounds), be prepared to treat it as a declared weapon and then be prepared to not be allowed to carry it at all if the agent doesn't understand that its harmless without rounds.
When you look into it you will find you can't carry hazmat on a passenger plane.