I realize that Kalifornia is our resident 'Nazi' state, but what about the federal law on 'assault weapons'.  If I am correct, there is no stipulation on whether a rifle has a forward pistol grip.  Right?  Now, on pistols, it DOES mention it.  

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I think so, but am not sure.  Just give the ATF the finger and get it anyway.
Ice

Yes, you can have a forward pistol grip on your AR, your Garand, or even your PSS if you want to. The ATF views this as just another means to hold the rifle with your support hand, just as if you were gripping the fore-arm. This year, at least...
It is illegal on a pistol, unless it is a machine pistol/smg (MP5K, etc.) or an AOW (semi-auto SP89 w/K-grip).
I don't know about the Kalifornia thing... I used to have a Knight's grip on my AR when I lived there a few years ago... Was told by an LA county sherrif's deputy what I just told you. 'Course, that was just before SB23 took effect.