IIRC you can have mags more than 10 rounds in CA if you had them before the ban.
There's was a big thread on this not too long ago.
Many range shooters sport standard cap magazines (16) and have no issue with them - I don't plan to load them in a HD gun, just for the range.
I'm fairly sure they won't get you on just the magazines, you'd have to do something else to get in hot water with LE's.
There's no law on buying, receiving or owning, so I should be in the clear if anything did arise.
As long as there are no manufacturer date codes on the mag, then no matter what, you are safe, as long as they dont catch you bringing it over state lines.
I have lots of hi-capacity mags and I live in CA. Hell, I even have some that say "For Military/Law Enforcement use only" BUT... They are still legal since I acquired them before 2000.
You can also technically acquire PARTS KITS to rebuild old mags. SO, if you have an HS2K and need to rebuild the mags, you could technically use XD Hi-Cap parts kits to rebuild your legal hi-cap mag without manfucturing a new high cap mag.
Anyway, tiger answered my question, thank you good sir