California Code, Penal Code - PEN § 17280

(a) After removal of grips, stocks, and magazines, the weapon is not as detectable as the Security Exemplar, by a walk-through metal detector calibrated and operated to detect the Security Exemplar.

(b) Any major component of the weapon, as defined in Section 922 of Title 18 of the United States Code, when subjected to inspection by the types of X-ray machines commonly used at airports, does not generate an image that accurately depicts the shape of the component. Barium sulfate or other compounds may be used in the fabrication of the component.