Source - https://oag.ca.gov/sites/all/files/agweb/pdfs/firearms/regs/text-adopted-regulations-bullet-button.pdf

California Code of Regulations
Title 11, Division 5

Any person who, from January 1, 2001, to December 31, 2016, inclusive, lawfully possessed an
assault weapon that does not have a fixed magazine, as defined in Penal Code section 30515,
including those weapons with an ammunition feeding device that can be readily removed from
the firearm with the use of a tool (commonly referred to as a bullet-button weapon) must register
the firearm before July 1, 2018.

Source - https://oag.ca.gov/firearms/bullet-button-assault-weapon

What is considered an “assault weapon”?

Pursuant to Assembly Bill 1135 (Stats. 2016, ch. 40) and Senate Bill 880 (Stats. 2016, ch. 48)
effective January 1, 2017, the definition of “assault weapon” is revised to mean the following:

RIFLES
a.  A semiautomatic, centerfire rifle that does not have a fixed magazine but has any one of the following:
 i.  A pistol grip that protrudes conspicuously beneath the action of the weapon.
 ii.  A thumbhole stock.
 iii.  A folding or telescoping stock.
 iv.  A grenade launcher or flare launcher.
 v.  A flash suppressor.
 vi. A forward pistol grip.
b.  A semiautomatic, centerfire rifle that has a fixed magazine with the capacity to accept more than 10 rounds.
c.  A semiautomatic, centerfire rifle that has an overall length of less than 30 inches.

It's just my opinion but I believe this to be correct and rational.

0.  Start with a semi-automatic, center fire rifle that is NOT an assault weapon (fixed ten round magazine).

1.  Opening the action and dropping the magazine does NOT create an assault weapon, it creates a rifle with NO MAGAZINE.

2.  Rifle can still be fed single rounds and will still function as a single shot rifle (a single round loading block facilitates feeding).

3.  Inserting a ten round magazine that locks in place does not create an assault weapon, it creates a rifle with a fixed magazine.

The section you cited says the following:

A rifle without a magazine by definition does not have a fixed magazine since it has no magazine.  If a semi-auto centrefire rifle has any of the features listed and does not have a fixed magazine, then it is an assault weapon.  That's if we accept the law as literally written.

I also found this in the AW regs [11 CCR 5471(m)]:

While obviously that doesn't apply to a rifle with a magazine catch, if it lacks a magazine it lacks a fixed magazine, and if it lacks a fixed magazine and has features, then it is an assault weapon.  This clarifies as well that for those two "styles" (however that's defined) of rifles  that the fact that a new magazine cannot be retained or attached without holding it in place due to a lack of a catch does not make the weapon something other than an assault weapon if it has features.

If a new magazine cannot be inserted and ammo placed in a position to feed from it and fire as the rifle is configured, it may be considered a single-shot.  Not sure exactly how the law applies in this context, particularly with ARs and AKs.  Under the old wording the lack of capacity to accept a detachable magazine would make this GTG.  Under the new wording it may not be.  I'm not 100% sure.

But if you can insert a magazine of any capacity into the weapon with the action closed and with an empty mag well, and it has features, that seems to almost certainly be an AW.  Even if not, better safe than sorry considering how LEOs can be in this State, especially in more anti-gun areas.

If they don't have a magazine catch assembly, the magazine can't be retained, - that is attached and/or detached intentionally.  The definition seems flawed on it's face.  Without the catch (assemblies) the mag will fall out and it won't feed if handled in ny reasonably normal way.  Likewise, it's an incomplete firearm.  Without a catch assembly, the firearm could be assembled, featureless and/or fixed mag - depending on the type.  It's like taking off the pistol grip or stock. , etc., until you put all of it together, you don't know what you have.

The regulations did unlawful things in other areas, like apply these rules to shotguns when the law didn't cover them, changed how length is measured for AW purposes, and other crap not in the law.  Those are the regs, and those are what will be enforced, unfortunately.

But it sounds like absence of a fixed mag may be sufficient to make a featured rifle an AW regardless.

When do the CARPA/NRA lawsuits start?

I would call them just CRPA lawsuits. NRA seems to tack on and take some credit more than anything else.

Not sure if the challenge to the AWB covers this or if there is a separate suit.  I'm pretty sure that something has already been filed or is in the works.  I know that some of the claims in the AWB case were dismissed the other day (I think that they were takings related).  CRPA has a lot of lawsuits going on right now.

Actually, it's quite possible to shoot an AR without a magazine catch, in particular when using a 10-round magazine.  The magazine is short enough, it mostly vanishes in the mag well, and can easily be held in there by hand. One would have to be careful to make sure it doesn't wiggle and misfeed, but most shots would work.  It is clearly a functioning gun that can fire shots, and it doesn't have a fixed magazine, therefore it fits the definition of "assault weapon" if it has evil features.

Sadly, your opinion that the regulations are unlawful, and my opinion, together with $5 will buy us a cup of coffee at Starbucks.  In practice, they are not unlawful until a court says so.  As you said, the regulations are enforceable until overturned by a court.

That court case was started over a year ago (to answer Trollslayer's question), and has already lost the first round in court.  Which is unfortunately quite typical of lawsuits filed by so-called gun rights organizations: they nearly always lose.  The reasons for that are debatable.  In my opinion, the main reason is that "our side" doesn't actually care about winning; they care about making an ideological point (that the 2A means all the guns, all the time, everywhere), demonizing the opponent, and stoking and serving the anger of their constituency.

But you are under no obligation to help them by demonstrating that in court.  Until completely assembled, parts is/are parts.

That's not how truth works.  Unless the law is altered, something either always was in compliance with the law, or always in violation of it.  Truth is not relative, not even in the context of law.  Relativism is popular among Leftist judges because it willadvance their causes, and DOJ does what it does because it believes (often correctly) that it can get away with breaking the law.  Doesn't change the fact that the law does not authorize their actions, which by definition makes themunlawful.

And the attorney in the court case clarified that they only had a portion of the complaint dismissed (the takings claim, but not the RKBA claim, IIRC), but are appealing what was dismissed and proceeding with what wasn't.

Since it is in the regulations, they obviously figured it out for themselves.