A 12-gauge flare gun could accommodate a 12-gauge shot shell but you run the risk of getting hurt when the plastic flare gun explodes with the powerful shotgun round. On the flipside, a 12-gauge flare can be fired out of a shotgun if it has absolutely zero choke, when you hack some of the barrel off. A standard barrel shotgun will most likely catch the flare payload in the barrel and burn inside, messing up the whole thing. It’s best not to interchange flare with live ammo.