.22 AR pistol

[essinem4130]

I've noticed not much info on these, what are the restrictions on them regarding barrel length, detachable magazine etc?

Can you use a NFA upper with a .22 bolt conversion without the registration hassle?

[CSACANNONEER]

What "registration hassle"? What's a "NFA upper"?

If you legally have an unregistered handgun in Ca, you don't need to register it to make a rimfire handgun out of it. If you legally own a lower which you can legally make a hanfddgun out of, I don't see what difference making a .22lr vs a 50beowolf handgun will make. If you are refering to less than 16" uppers as "NFA uppers", then, yes, you can use a <16" upper to manufacture a handgun. But, many people also use them to legally produce title one rifles as well just by permanently attaching something to them to make length. I would consider <16" barrelled upper more as handgun uppers than "NFA uppers".
Most ARs are .22 cal but, if you mean .22LR, California laws regarding AWs do come into play for handguns. That means, unless you have a RAW lower which can be legally built into a handgun, you need to comply with all the CA AW laws. That means, no detachable mag outside the PG, no +10 round fixed mag, etc. I will suggest that you read the laws for yourself and, maybe look at the handgun AW flow chart too.