California Code, Penal Code - PEN § 16530

(a) As used in this part, the terms “firearm capable of being concealed upon the person,” “pistol,” and “revolver” apply to and include any device designed to be used as a weapon, from which is expelled a projectile by the force of any explosion, or other form of combustion, and that has a barrel less than 16 inches in length. These terms also include any device that has a barrel 16 inches or more in length which is designed to be interchanged with a barrel less than 16 inches in length.

(b) Nothing shall prevent a device defined as a “firearm capable of being concealed upon the person,” “pistol,” or “revolver” from also being found to be a short-barreled rifle or a short-barreled shotgun.