I HAVE AN ASSAULT PISTOL AR-15 AND I PUT A  PISTOL STYLE FORWARD GRIP ON IT.IS IT AN ILLEGAL THING TO DO?ANY REFERRENCES TO CALIFORNIA LAWS?THANKS.

Think also about whether it is an AOW under U.S. law.  Anyone weighing in on the subject, what would the answer be if it were a pre-ban [u]rifle[/u] (not SBR)

No can do. By adding a vertical foregrip, you would be making an AOW ("Any Other Weapon") subject to the National Firearms Act.  Theoretically you could do it, but you would first have to submit a form 1 with pictures, fingerprints, CLEO signoff, and $200 making tax to the BATF NFA Branch. After you received approval you could add the grip. However, since it looks like we're talking about CA here, I don't think they're permitted under state law, and there's no way you get the CLEO signoff.

If it were a preban rifle and he merely wanted to add the vertical foregrip (and keep the stock and barrel length the same), there would be no problem.

Thanks a lot for info.