There is some confusion on that issue, and the laws may change soon. But as far as I know the current bullet button is NOT illegal to use, and the DOJ has approved a bullet as a tool. This site has a very good section regarding the laws, without it I would never be able to keep up with the laws in CA because the laws change every few months.

http://www.calguns.net/copmemo2.pdf

I still wouldn't trust any legal advice I find on the internet, but at least I can keep up with the new changes and it can lead me to the information I'm looking for.

For some it's a grey area, and not worth the risk to get hassled by an uniformed LEO. So some people (like me) don't use it and just use a mag lock kit instead. I'm just pointing out that the AR-15 is NOT illegal in CA, and you're not limited to 10 round fixed mags.

Edit: Looks like Arney beat me to it. Like I said, don't believe most of what you read on the internet, but you have to do the research for yourself.