Build the rifle you want and put this stock on it.
www.californiarifles.com
I checked out the site... looks good but the one thing I DON'T see that would have given me warm fuzzies is a determination letter from the CA DOJ that a semi-auto, removable magazine rifle with this stock is not considered an "assault weapon" under California law.
Since this outfit is in Sacramento (my own AO) I might have to truck on over there and have a chat with them
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I am one of the early 5 or 6 "sh*t disturbers" in the California off-list AR gig so I know some of the background here... It is unlikely that with current DOJ staffing, and the fact that the current director (in 'retirement mode' with an apparently ailing hip) is letting the antigun Deputy AG (Firearms) run the division from the Brady campaign, that anything stating "this is legal" will emerge unless/until staffing changes.
This stock is clearly legal by statutory and regulatory law (12276.1PC / 11 CCR 5469). The web of the hand cannot go below the top of the trigger no matter how fat your hands are, and it cannot be considered a thumbhole stock either.
An alternate non-pistol-grip grip device called the MonsterMan grip works with A2 stocks. This grip is not described by the 11 CCR 5469 regulatory definition of pistol grip either and thus does not trigger the definition of AW if no other evil features present. Some DOJ phone staff even are informally recommending the MonsterMan grip (not by name); I don't think they know about Grant's U15 stock yet.
Also, a letter from DOJ is useable as an element in a defense. But you should know the law as your primary basis for legality, as a DOJ letter is not necessarily a guarantee like a formal opinion letter is. The DOJ has in fact issued such letters that have backfired: in one case, they told folks with AWs that were permitted that they did not need to register them when SB23 rolled around. This person was later popped with a DOJ-permitted, but not registered, AW, and was found guilty. (Have no idea about reversal on appeal).
One famously incompetent and undersupervised CA DOJ special agent "Iggy" even accidentally approved a vendor's welded-up fixed-mag *named* assault weapons, because he did not understand what "series status" meant or how it was irrelevant to the matter at hand. There was a DOJ 4+ hr senior staff meeting regarding the cleanup of this issue. and some kinda funky permits may go out to 'fix' this situation (DOJ approval of an illegal named AW).
Understand the tech details of the law and work with that. Talk to folks who have worked these things out (and talked to lawyers etc) The best discussion on CA firearms legalities is on Calguns.net rifleman's forum and 2nd Amend/Legal/Political forum.
Bill Wiese
San Jose CA