assault weapom?
Are you sure that the reason it is no allowed in CA is because it is an assault weapon?
Perhaps it is just because it is not on the California approved handgun list.
This might be due to many factors, including it did not pass the drop test, it was not submitted to the drop test, or it passed or did not pass and the manufacturer did not want to bother to pay the LISTING fee for that particualr model.
Each particular model must be listed.
What makes you think that the assault weapon definition was the reason?
there is a Walther P22 CA version on the list. Not sure of the difference, but probaly only difference is the ten round mag.