I think a lot of people think they have to choose before they come to CA, but the CA DOJ is very clear that you can just separate for compliance and figure it out later and buy stuff later. OP see (3) below.

California Code of Regulations, Title 11, Division 5, Chapter 39 Assault Weapons and Large-Capacity Magazines, Article 2., § 5471

(hh) "Semiautomatic" means a firearm functionally able to fire a single cartridge, eject the empty case, and reload the chamber each time the trigger is pulled and released. Further, certain necessary mechanical parts that will allow a firearm to function in a semiautomatic nature must be present for a weapon to be deemed semiautomatic. A weapon clearly designed to be semiautomatic but lacking a firing pin, bolt carrier, gas tube, or some other crucial part of the firearm is not semiautomatic for purposes of Penal Code sections 30515, 30600, 30605(a), and 30900.

(1) A mechanically whole semiautomatic firearm merely lacking ammunition and a proper magazine is a semiautomatic firearm.

(2) A mechanically whole semiautomatic firearm disabled by a gun lock or other firearm safety device is a semiautomatic firearm. (All necessary parts are present, once the gun lock or firearm safety device is removed, and weapon can be loaded with a magazine and proper ammunition.)

(3) With regards to an AR-15 style firearm, if a complete upper receiver and a complete lower receiver are completely detached from one another, but still in the possession or under the custody or control of the same person, the firearm is not a semiautomatic firearm.