Sorry to bug but for some reason I can search the hometown forums as I figure this has been answered a million times.

I'm trying to help a friend in CA figure out the specifics on the laws.

What were trying to clarify is if he builds a featureless AR can he run a detachable 10 round magazine? or do all AR's require fixed magazines?

Thanks in advance,
George

If it is truly featureless (I assume a rifle, as there is no way to do a featurless AR pistol) and its OAL is at least 30 inches in its shortest configuration then he can use detachable magazines and they can be of any capacity.

Featureless means zero features, obviously.  No rifle grenade spigot or anything else considered a grenade or flare launcher, no flash suppressor, no vertical and I think even angled foregrips, no pistol grip, no thumbhole stock, no collapsible or retractable stock, and no folding stock (not sure this applies if it cannot be fired with the stock folded or can only fire one shot like the Law tactical folder).  Any one feature renders it an AW if it lacks a fixed magazine (the previous language was if it had a detachable magazine, but the new laws changed things and are more stringent).

As an aside, fixed mag with greater than 10-round capacity makes it an AW regardless of features.  OAL in shortest configuration (by law I think it includes the muzzle device regardless of permanence, but DOJ has been trying to assert and incorporate regulations definitions that measure it with the muzzle device removed despite no change in the statutes in that regard) under 30 inches makes it an assault weapon regardless of features.  Under 26" in shortest configuration or under 16" barrel makes it an SBR, additionally.

If it is a rimfire AR, then I don't believe any of this stuff applies (for sure the feature stuff doesn't apply; I'd have to look up the rest to be sure).

AR pistol, again, has to have a fixed 10-round or less mag (because an immutable feature is a magazine well outside of the pistol grip, and only one feature is required to be an AW) and can only be legally acquired if it is on the safe handgun roster, acquired via an exemption to the roster requirement via PPT (buying from some LEOs, intrafamilial transfer, the latter), is acquired using the single shot exemption (some people think conversion to semi-auto is no longer legal, but others do not and I am in the latter camp), or is built using the single shot exemption and going through the process to get permission and a serial number for a home built firearm in which the receiver is registered as a pistol.   If a weapon or receiver DROSed or registered as a rifle is used to make an AR pistol, it is considered an SBR in CA.  AW laws for pistols apply to rimfires, too, unlike for rifles.  Braces are a bit of a grey area.  Not explicitly illegal and DOJ seems to tolerate most of them, but shouldering in a manner that wouldn't upset the ATF is questionable and the DOJ might not like it.  They did approved braced pistols for AW registration, though, when the registry was still open.

Thank you for the very descriptive response. Sounds like he's in good shape then. here's his plan.

Moe A2 stock, one of those fin grips, free float rail with nothing on it, bare muzzle, 16.1" barrel, it will be a scoped only rifle.

Yeah, should be GTG with detachable mags of any capacity.  There are some better grips that are not considered pistol grips than the ones with the fins.  Finned grips kinda suck if you're doing anything more than shooting from a bench.  He can also do a muzzle brake or compensator that are not classed as flash suppressors (don't reduce flash or direct it away from the shooter's vision, aren't advertized or designed to do such, and aren't called flash suppressors) if he wants.  Some people do play it safe and do the plain muzzle or thread protector route, though.

For my dad's 16-inch AR that I built for him I used a Resurgent Arms grip, Magpul stock (which doesn't work the best with the grip, and I've suggested fixing the collapsible stock I put on there when I built it as a bullet-buttoned rifle), and a Griffin Armament M4SD linear compensator.