Carry it in your vehicle like you would any other firearm as per State law then as you're right, it could/would probably be technically classified as a firearm:
167.31(c) "Firearm" means a weapon that acts by force of gunpowder.
175.60(bm) "Handgun" means any weapon designed or redesigned, or made or remade, and intended to be fired while held in one hand and to use the energy of an explosive to expel a projectile through a smooth or rifled bore. "Handgun" does not include a machine gun, as defined in s. 941.27 (1), a short-barreled rifle, as defined in s. 941.28 (1) (b), or a short-barreled shotgun, as defined in s. 941.28 (1) (c).
On the other hand, the ATF doesn't consider a flare launcher to be a firearm by itself. It will, however, become classified as AOW if you have inserts installed to fire standard cartridges.