California Code, Penal Code - PEN § 16525

(a) As used in Section 33700, “firearm barrel” means the tube, usually metal and cylindrical, through which a projectile or shot charge is fired.  A firearm barrel includes any forging, casting, printing, extrusion, machined body, or similar article that has reached a stage in manufacture where it may readily be completed, assembled, or converted to be used as a firearm barrel, or that is marketed or sold to the public to become or be used as a firearm barrel once completed, assembled, or converted.  A firearm barrel may have a rifled or smooth bore.

(b) A firearm barrel that is attached or affixed to a firearm is not subject to the requirements of Section 33700.