The term "any other weapon" appears just once in the entire firearms section of the code, as a valid exemption to 12020 prohibitions. And AOW does not appear at all. No place do I see anything that CA specifically states that a federally registered AOW is illegal in CA. So, you would need to look at what features of an AOW that would be banned by CA. Stuff like smooth-bore pistols, unconventional firearms, and other stuff.
So, the feature you are talking about adding a VFG to to semi-auto detachable mag pistol. Doing that would normally be creating an AW, which would be illegal unless registered. But your SP89 is already a reg'ed AW, so what CA law did you violate by placing an VFG on it?
I think if you could get a CLEO sign-off on your Form 1, you should be golden. Send it to the BATFE and it should come back approved in a couple months. After you get the AOW approval from the feds and install the VFG, what crime would you have committed in CA. You didn't make an unregistered AW, it already was registered. Oh, and ATF will not approve any app that violates state law.
Remember that CADOJ does spread FUD if they don't like what we are doing. So, their opinion may not follow the letter of the law. Just look at the entire OLL deal.
Do you want an AOW or C&R SBS/SBR in CA?
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No posts of mine are to be construed as legal advice, which can only be given by a lawyer.