Actually, unless a .50 cal assault rifle is specifically named on the CA DOJ list by model and part number, it is technically legal to own as long as it meets certain criteria. If the rifle is equipped with "evil features", then it must have a bullet button magazine lock installed, and the rifle can use no more than a 10 round magazine. This also goes for AR style rifles as well.
"evil features" are: pistol grip, collapsible stock, fore grip, et al.
Please refer to the link below for questions on owning assault rifles and hand guns in California.
www.calguns.net/caawid/flowchart.pdf
Legal to sell and legal to own are two different things.
"please do not construe what I have told you as legal advice, I am not a lawyer"