The Roberti-Roos Assault Weapons Control Act of 1989 banned Colt AR-15 rifles by name in the State of California. California's 2000 Assault Weapons ban went further and banned AR-15s made by other manufacturers by name such as Bushmaster, PWA, and Olympic Arms.
AR-15s in California Wikipedia
This style of rifle is made by combining an AR-15 upper receiver with an AR-15 lower receiver which has not been banned by specific name, and which has a fixed, non-detachable 10-round (maximum, anything above 10 is a felony) magazine. In such a configuration, otherwise prohibited features such as a telescoping stock, pistol grip, and flash hider may be present. While formerly prohibited under the now-expired federal assault weapon ban of 1994–2004, the presence of a bayonet lug is not prohibited by California state law and can be present on firearms without violation. However, the magazine cannot be detachable, so to load the rifle the shooter must either "top load" or use a tool to release the magazine lock (such as the tip of a bullet in "bullet button" equipped rifles). To top-load, the shooter pulls the rear takedown pin, hinges the upper receiver on the front pivot pin, and loads the now exposed magazine. Alternatively, several "magazine lock" devices are available which replace the magazine release button with an inset pin that requires the use of a special tool to release the magazine, thereby, disallowing a readily "detachable magazine", to be compliant with California state firearms law.
If a rifle has none of the prohibited features (pistol grip, telescoping or folding stock, flash hider, grenade/flare launcher, forward pistol grip), the rifle may be used with legally owned large-capacity magazines. There are several products available on the market to configure featureless rifles. The California DOJ Bureau of Firearms has attested under oath that the U-15 stock, the Hammerhead Grip and the MonsterMan Grip do not constitute a pistol grip and are therefore legal when used on a detachable magazine semiautomatic centerfire rifle with none of the features listed in CA PC 30515(a)(1)(A-F).
Most AR-15 manufacturers now make lower receivers which qualify as "Off-List" Lower (OLL) receivers which are legal to possess and use in the state of California. With regard to standard capacity magazine devices, after January 1, 2000, it is illegal to offer for sale, import, manufacture, give, or lend (although it is legal to possess and use) any detachable box magazine with a capacity exceeding 10 cartridges. Individuals possessing such magazines at a lawful location such as a gun range may only allow use of their lawfully owned standard-capacity magazines by another if they are in the immediate vicinity of the person using such items. All magazines acquired after (and including) January 1, 2000 must be a limited capacity of 10 rounds or be in felony possession of an illegal standard-capacity magazine.