not a %100 sure, but 1) it sounds like youre in california, and 2) one thing i have heard on the subject is if you a SBR upper, and no pistol lower to match they could nail you with intent to assemble an illegal SBR. not quiet sure how the registering of a pistol lower works though. I have a 9mm AR and was throwing the idea around on whether to make it a pistol or not when i first set out to build it, and decided to just go with a full size carbine.
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