Discharging a BB gun is illegal in the City of Atlanta. That still doesn't make it a firearm. Same goes for shooting a bow and arrow in my backyard. I suspect this is the case in many other municipalities.
In Coweta County it's illegal to shoot BB gun, pellet gun, crossbow, bow & arrow or anything that launches any type of projectile capable of causing bodily harm within 300 yards of a neighbors property line (not house but property line) unless you have permission from the property owners. We had a problem in our neighborhood years ago with an idiot shooting at peoples critters.