Now cite a case in which a court found that a State's exercise of its police powers to regulate possession of something within its borders constituted a regulation of commerce.
That would be Brown v. Maryland - 25 U.S. 419 (1827)
where the State of Maryland used it's police power to require a license for the importation and wholesale of " foreign goods by the bale or package, &c.".
Further, we have here
a paraphrase or quote, I'm unsure which as I can't seem to find the text in an opinion, or even the case referenced,
In the very first decision under the commerce clause Chief Justice. Marshall established that a state police regulation in conflict with a congressional regulation of interstate commerce is void.
AND we have 18 USC § 922
Originally Posted by US Code
§922. Unlawful acts
(a) It shall be unlawful—
(1) It shall be unlawful for any person to manufacture, import, sell, ship, deliver, possess, transfer, or receive any firearm—
(A) that, after removal of grips, stocks, and magazines, is not as detectable as the Security Exemplar, by walk-through metal detectors calibrated and operated to detect the Security Exemplar; or
(B) any major component of which, 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
Between that, NFA, and other regulations we have ample examples of the Government exercising it's commerce power over which firearms are legal and not legal in interstate trade. The police power of California appears in conflict with that by being more restrictive.