Whoa, lets clear up some misinformation here.
You can in fact build an AR type rifle in CA. You can build the rifle one of two ways, either with all the "evil" features (flash suppressor, pistol grip, collapsible stock, etc.) and pinned 10 round magazine
OR
Detachable magazine using 30 round or whatever capacity mags you want, BUT no evil features like pistol grip, flash suppressor, etc. Basically in this configuration you have a rifle with a detachable magazine, but winds up being pretty tough to shoot since it doesn't have a pistol grip.
If it were me I'd elect to build a pinned mag rifle that is essentially a top loader that has to be broken open to be loaded. This way at least when I travel to Arizona or Nevada for a shoot, in less than 5 minutes I can remove the dumb magazine lock and have a fully functional AR. Reconfigure to CA legal status with mag lock attached and 10 round mag back in place, and you are ready for the trip back across state line behind the iron curtain.