Cal. Penal Code § 16885

(a) As used in this part, “machinegun-convertible pistol” means any semiautomatic pistol with a cruciform trigger bar that can be readily converted by hand or with common household tools, as defined in Section 4082 of Title 11 of the California Code of Regulations, into a machinegun by the installation or attachment of a pistol converter as a replacement for the slide’s backplate without any additional engineering, machining, or modification of the pistol’s trigger mechanism.
(b) A machinegun-convertible pistol does not include a hammer-fired semiautomatic pistol or striker-fired semiautomatic pistol lacking a cruciform trigger bar, which instead has a trigger bar that is shielded from interference by a pistol converter.
(c) A polymer notch or other piece of polymer molded into the rear of the pistol frame does not prevent ready conversion into a machinegun and will not prevent a pistol from qualifying under this definition.