Not quite true and the "Constructive Possession" discussion is not complete "FUDD."
To start with, you are completely correct that California's "Assault Weapon" statute does not include a "Constructive Possession" clause. But please do not be so naive as to believe that the lack of a "Constructive Possession" clause makes such possession legal. Remember that this is California, and the prosecutorial legal shenanigans run quite deep.
Where your argument goes into the toilet is with the California Court of Appeals 2013 decision in People v Nguyen. That case started when Mr. Nguyen was caught with the unassembled component parts of an Assault Weapon. The nice Orange County District Attorney charged Mr. Nguyen with the "Attempted Possession of an Assault Weapon." The DA could not charge the primary offense because the parts were unassembled, and (as you have pointed out, the AW statute does not include a constructive possession clause), the elements of the primary offense were not present. But Penal Code section 664 makes the attempt to commit the primary offense punishable, and that's the route that the nice District Attorney took. Mr. Nguyen was convicted and sentenced to six years.
The Court of Appeal basically found that there was no innocent purpose to the possession of the component parts other than to a some future point, assemble them into an Assault Weapon, and on that basis the attempted crime was complete.
The court gave an interesting treatment to Mr. Nguyen's attempt to argue the "Rule of Lenity" in his defense. In simple terms, the "Rule of Lenity" provides that if the ambiguous facts of a case can be added up to an innocent conclusion, and also be added up to a guilty conclusion, a criminal court must add them up to the innocent conclusion. In the Nguyen decision, the court discounted the possibility that Mr. Nguyen had an innocent purpose to the possession of the AW parts - "the rule of lenity applies only if the court can do no more than guess what the legislative body intended; there must be an egregious ambiguity and uncertainty to justify invoking the rule.‟
So, at the end of the day, even though there is no "Constructive Possession" clause in California's AW statute, Mr. Nguyen went to prison for six years for what was essentially the constructive possession of an AW.
You’re conflating two different concepts.
I already acknowledged that PC 664 can support an attempt theory. Nobody is saying the absence of a constructive-possession clause gives someone immunity if the evidence shows intent plus a direct step toward manufacturing or possessing an illegal assault weapon.
But Nguyen was not a generic constructive-possession case. It was an attempt case with terrible facts. He had an AK parts kit, had taken assembly steps, and, most importantly, admitted to law enforcement that he intended to build/possess the illegal assault weapon. That admission is doing a lot of work in the case.
Attempt requires specific intent plus an act beyond mere preparation. Nguyen had both. That is very different from someone merely owning lawful feature parts that have lawful uses.
Calling Nguyen “essentially constructive possession” muddies the issue. The prosecution could not prove actual possession of a completed AW, so it proceeded under attempt. That does not create a general rule that loose parts automatically equal possession of an assault weapon.
The correct statement is narrow: parts plus admitted intent plus conduct beyond preparation can support an attempted AW theory under PC 664. The incorrect overbroad forum version is that California has a general constructive-possession doctrine for AW parts. It does not.
So yes, Nguyen is a cautionary case. No, it does not mean a pistol grip, flash hider, upper, stock, or other lawful component sitting unassembled in a parts bin is automatically criminal. The legal issue is the totality of facts showing attempted manufacture or attempted possession, not mere ownership of parts.
Also worth noting: Nguyen was not some clean-record guy who accidentally had a grip and flash hider in a parts bin. He had a prior juvenile aggravated-assault adjudication that was treated as a strike, plus the AK kit/partial assembly facts, plus statements showing intent.
That does not make the case irrelevant, but it makes it a very poor case to generalize into “loose parts = constructive possession.” Nguyen is basically the nightmare fact pattern, not the ordinary lawful-owner fact pattern.