California Code, Penal Code - PEN § 16531

(a) “Firearm precursor part” means 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 the frame or receiver of a functional firearm, or that is marketed or sold to the public to become or be used as the frame or receiver of a functional firearm once completed, assembled or converted.

(b) The Department of Justice, consistent with this section, shall provide written guidance and pictorial diagrams demonstrating examples of firearm precursor parts.

(c) Firearm parts that can only be used on antique firearms, as defined in subdivision (c) of Section 16170, are not firearm precursor parts.