We are not attorneys and are not giving legal advice, but we want to help explain to you how we understand the firearm law in California.
For centerfire rifles in California the "assault weapon" regulations require one to make certain changes to their firearm for compliance. CA law looks at your features (pistol grip, retractable stock, flash hider, thumbhole stock, and forward grip) in combination with a detachable magazine to determine if your rifle is an "assault weapon" or not.
If you have a detachable magazine and any one of the above mentioned features in CA, then you have an assault weapon in CA. Compliance in this state would then be done by either removing the features or the detachable magazine. These can both be done fairly easily.
Removing the features can be accomplished by:
1.Replacing your pistol grip with a fin grip.
2.Having a non-adjustable or pinned stock that does not adjust. In the case of a thumbhole stock, you would need to replace it.
3.Having a muzzle break instead of a flash hider.
4.Not adding a forward grip.
If this is not a good option for you, you can keep all the features by removing or "fixing" your magazine so it is no longer "detachable"
A CompMag is a good option in this case. It locks into your rifle and gives you a "fixed magazine" under CA law. CA law states that to have "fixed magazine" one would need to "break the action" of the firearm to remove the magazine. As the CompMags cannot be released without separation of the upper and lower, this would qualify as "breaking the action".
The CompMag is easily loadable through the side of the magazine, so reloading your firearm without removing the magazine is not a problem. It is also is reversible should you decide to shoot in a free state.
There are also rifles on the "ban list" of firearms that are automatically deemed "assault weapons. For these, there is no fix.
Here is a 2017 Flowchart from the CRPA to help you determine if your Rifle or Pistol is California Compliant. Please read the "Quick Reference Guide" below that, it has a lot of good information you should know to help keep yourself safe.
This is not legal advice. Consult with a qualified attorney to account for your individual circumstances.
This Flowchart will help gun owners who own firearms not already registered as "assault weapons", or not already prohibited by make/model.
A complete list of firearms which are prohibited by make/model can be found in the California Code of Regulations or in the "Assault weapons identification guide" on DOJ's website here:
https://oag.ca.gov/sites/all/files/agweb/pdfs/firearms/forms/aws-guide.pdf?
Page 58 for AK rifles, Page 63 for AR rifles.