I beleive that any semi automatic firearm with detachable magazine is illegal in California. AR platform firearms sold in California are equiped with a magazine release the requires a tool to release the magazine, the so called bullet button. There are other restrictions as well, in that I also believe that there is a list of prohibitied firearms that you should consult. Any firearm with bushmaster in its name is going to draw attention.
I suggest contacting the California State Rifle and Pistol Association,
http://www.crpa.org/ or perusing the California AG's website. I would give a link, but everyone I tried was spectacularly uninformative.
Here is a summary from Wikipeida that may be applicable to your firearm (
http://en.wikipedia.org/wiki/Gun_laws_in_California):
. . . the following firearms are banned by characteristic (from Penal Code §12276.1):
(1) A semiautomatic, centerfire 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 thumbhole 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, centerfire rifle that has a fixed magazine with the capacity to accept more than 10 rounds.