Legal Question in Civil Rights Law in California

Highly Doubtful. Get rid of it (destroy it or take it out of state) or you could do hard time just for possessing it. In most other states it's regulated (or not regulated) the same as any other semiautomatic rifle.

Before assuming anything, you first need to determine whether THIS gun is classified as an 'assault weapon'. Not all AR15's are. Talk [anonymously] to a gun dealer knowledgeable about AR's, and be prepared to describe the make, model, serial number series, regulated features, etc, to see if registration is/was required, and whether the gun can be modified to remove it from registration requirements. Many times that is possible. Do NOT take the gun to any dealer to examine. They don't want legal problems either.

IF the gun is subject to 'assault weapon' registration, then:

If you purchased it after the 'assault weapon' ban / registration was imposed, you would have been required to do all the necessary paperwork at the time of purchase. If instead you are talking about a gun bought before the ban / registration was imposed, which gun was then supposed to be 'voluntarily' registered in accordance with the rules imposed, you had a limited time to get it registered. It is too late now to do so, and you are in possession of an 'illegal' weapon for which you would be put in prison if caught and convicted. Your options are to keep it and risk the consequences, or take it discretely out of state for storage or legal sale.

If you get caught with it in CA, feel free to contact me for the criminal defense you are most certainly going to need.