Being a California resident I can speak to this...
- You need an off list lower if not registered as an assult rifle before the ban.
If you want to have the evil features you need the bullet button. In addition your magazine can not exceed 10 rounds.
If you want a detachable magazine you can't have any evil features and your magazine can not exceed 10 rounds.
The relevant section of California Penal Code
12276.1 (a) Notwithstanding Section 12276, "assault weapon" shall also mean any of the following:
1. A semiautomatic, centerfire rifle that has the capacity to accept a detachable magazine and any one of the following:
1. A pistol grip that protrudes conspicuously beneath the action of the weapon.
2. A thumbhole stock.
3. A folding or telescoping stock.
4. A grenade launcher or flare launcher.
5. A flash suppressor.
6. A forward pistol grip.
Once you install the bullet button on the lower it no longer has the ability to accept a magazine and therefore the subsection no longer apply--IMHO.
(DISCLAIMER--I'M NOT A LAWYER, THIS NOT LEGAL ADVISE)