It's a complicated question, and I'm not a lawyer, but I can give you an overview.
The AR-15 rifle receives tighter restrictions in California than elsewhere, but they are not against the law in themselves. Basically, one has to follow certain restrictions.
- AR-15 rifles purchased before 2000, and registered, are "Registered Assault Weapons" in California, and these weapons can have features that aren't legal on weapons purchased after that date. These are the only weapons allowed to carry the AR-15 name on the firearm itself.
- 10 round capacity. Anything more is a felony (unless you're in an exception group like certain armored car drivers). Exceptions are made (see #4)
- The magazine cannot be "readily" detachable, though it is legal via a "bullet button" or other unlocking mechanism that slows things down.
- If the weapon has no "accessories," such as telescoping stock or flare-launcher, and was purchased before 2000, it is legal to use extended magazines that were in the owner's possession at that time. You are not allowed to acquire new ones for use, even if you have a registered weapon, unless you fall into a certain category of people who use the weapon as part of their job.
- If you own a registered AR-15 with extended magazine, you may only allow someone else to use the extended magazine at a lawful location, such as a shooting range, and only as long as the magazine stays within your sight.
- A bayonet "lug" does not count as an "accessory."
That's about it, but again, a gun store owner in California could probably fill you in a little better.