for now, many jurisdictions do not consider blackpowder guns to be 'firearms', and I'm ok with that.
But JimK makes a good point. Laws have a way of being fairly vague and not as definitive as they could or should be.
A tin cup can be a deadly weapon(if you saw Riddick), but it's definitely not a firearm.
My definition of a firearm, is an arm/weapon that utilizes some form of fire.. such as how gunpowder combusts to propell a bullet, or the way two chemicals mix to make fire in flame-throwers.
They're firearms.