Not looking for an argument or talking about defaced firearms, but a nuance missed by some people is the supremacy clause. States can make laws which are more restrictive than federal law, but not less restrictive, EXCEPT when the more restrictive state law violates a constitutional right.
So IF the Braun decision referencing "historical tradition of firearm regulation" applies to home made firearms and those firearms did not require serial numbers under the definition of "historical tradition..." (because it's been that way forever) then California passing a law that home made firearms have to have serial numbers applied violates your constitutional rights, according to the Braun decision. At least one could make a valid argument.
Cali is anticipating a ton of lawsuits over their anti 2nd laws, which is why the are passing (or have passed) that if the plaintiff on any 2nd amendment lawsuit against the state fails in any of it's arguments, they have to pay court fees and damages to the state. Clearly this violates the 1st Amendment, but that's another story.
No, I'm not a lawyer, but my wife and daughter are, so take pity on me.