What is listed as illegal in CA is a: "Bushmaster assault rifle" and "Bushmaster: XM15 (all)"
I don't know where your model in particular falls if it's in either of those two categories or not. Someone else that is familiar with Bushmaster might be able to help you better.
IF, your receiver is not considered to be on the list, than ALL you need to do is get a bullet button, or any of the other types of AR15 mag locking device, install it on the rifle so that your magazine can NOT come out of the gun without a tool to remove it. Then make sure non of your mags can hold more than 10 rounds. You should be CA legal as far as I know.