All AR15s have a front and rear "takedown pin", including those sold in California. The rear pin can be pushed out to "shotgun" open the receiver.
The "bullet button" was a work around devised to bypass the FORMER version of the CA assault weapon ban. The bullet button is no longer a valid work around, and is not included in the purchase of any new AR15 in California.
The CURRENT version of the CA assault weapon law is still features related with a new functional work around - which may be what you are referring too with the rear takedown pin.
Under the current - as yet not overturned law due to an appeal to the 9th Circuit, An "AR15" pattern rifle must not have a flash hider (brakes are fine), no bayonet lug, no pistol grip protruding conspicuously below the receiver, a fixed stock, and a FIXED 10 round magazine that can only be removed by separating the upper and lower receiver.
The work around for the pistol grip is to install a "fin grip" to the pistol grip which prevents the palm and thumb from passing around the grip, thus converting it into a non-grip per se. Combined with the removal of the aforementioned items, this alone allows the use of a conventional magazine release button - no "bullet button required at all." The FIN is usually a panel of Kydex formed around a standard pistol grip and anchored by screw rivets - easily removed, restoring the rifle to original functionality.
For those who wish to keep a proper pistol grip, the magazine must be fixed in place, and the work around to this is the installation of some version of a spring-loaded button to push the rear takedown pin partially out which allows the upper to open a tiny crack, being retained by a flat section on the rear pin. The bolt hold open is replaced with one designed to block the magazine release from actuating - it overhangs the release bar on the left side. When the receiver is cracked open, this block raises sufficiently to allow the standard magazine release to be operated. The DOWNSIDE to this setup is the bolt can no longer be locked open and so the replacement hold open omits this capability. Were the bolt locked open, the receivers could not separate, so the bolt closes on the last shot. This also makes it impossible to lock the bolt back to clear a jam, or to remove the magazine if the bolt carrier is partially open as this bridges the lower receiver ring. This looks like a nifty solution until one considers the loss of functionality that cannot be readily restored without tools, time, and knowhow.
Another work around is a unique pistol grip that juts straight back so as to elevate the thumb above the line of the trigger forcing the hand into a position akin to that of gripping a conventional rifle stock. Goofy, but workable.
Of course "featureless" is still possible as this miraculously transforms the AR15 into a completely harmless, socially responsible rifle by replacing the entire pistol grip and stock to position the hand around a conventional rifle style grip shank, and of course no flash hider, bayonet lug, etc.
Also, any standard AR15 pattern rifle MUST be appropriately serialized and registered with the State under the NEW version of assault rifle registration scheme. Since all rifles not "fully featured" are considered AR15s, they do not require registration.
The U.S. Fifth District court just issued yet another ruling that the CA assault rifle ban is unconstitutional, and said ruling is under a 10 day stay, awaiting review by the 9th Circuit which will almost certainly extend the stay awaiting review by the full court. Despite the Bruen ruling, a full court panel will almost certainly side with California, but this time it will be a HUGE mistake because then it will be appealed to the Supremes and this is the case they've been waiting for since Bruen. By refusing to surrender to defeat - hoping the political climate will shift and the court will shift before this case gets there, California risks being THE State responsible for ending ALL assault weapons bans nationwide.
There are similar cases in other districts and circuits, so the race is on to see which gets to the Supremes first to settle the issue once and for all - until a future SC flips the script.
