BB guns don't exist as a classification in the CCC. You're either at/over the fps and muzzle energy limit, which made you a firearm, or under the limit, which made you an imitation firearm. If you're under the limit, what matters is your appearance. Replica firearm classification ignores the device's ability to fire projectiles, unless the device meets the required fps and ME to be a firearm. Then the device is straight up a firearm.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."