Thanks for your question.At the present time all assault weapons are illegal to buy in California--fortunately they define assault weapon rather tightly. Several firearms are listed by model and manufacturer and a genXXXXX XXXXXst of characteristics is included. A list of specifically prohibited firearms is available at the California DOJ website. I will include the list of features taken from the actual statute. Several makers currently offer California legal AR type weapons. You can also buy an "off list" receiver and put one together yourself so that it does not violate the current law. The key is to make sure that your semi-automatic center fire rifle does not have a magazine that is removable without the use of a tool. Have fun and good luck.
12276.1. (a) Notwithstanding Section 12276, "assault weapon" shall also mean any of the following:
(1) A semiautomatic, center fire rifle that has the capacity to accept a detachable magazine and any one of the following:
(A) A pistol grip that protrudes conspicuously beneath the
action of the weapon.
(B) A thumb hole stock.
(C) A folding or telescoping stock.
(D) A grenade launcher or flare launcher.
(E) A flash suppressor.
(F) A forward pistol grip.
(2) A semiautomatic, center fire rifle that has a fixed magazine with the capacity to accept more than 10 rounds.
(3) A semiautomatic, center fire rifle that has an overall length of less than 30 inches.
The list of prohibited weapons can be found at the following link as well.