I got this response from CHP. They didn't exactly address the magazine capacity but did say your issued duty weapon was ok.
Good Morning,
I cannot verify the information regarding two out of state law enforcement officials being arrest for the capacity of magazines, but the important thing when traveling to another state is having your official identification available if you are contacted by law enforcement. The unfortunately part is many contacts we have with out of state law enforcement would have been much simpler if the off duty officer had more than just a badge available as identification of peace officer status. In California, the badge has to be backed up with a photo identification card issued by the officer's law enforcement agency, indicating the person's peace officer powers are granted under the issuing state's governing codes. It may seem frustrating but there are many security companies and other parties who try to impersonate law enforcement using just a badge or generic ID card, so we have to be very careful when verifying out of state credentials. If you are a peace officer with proper identification, you should not have an issue carrying your issued duty weapon into California. Some counties have more specific restrictions on firearms, but again, with proper identification, peace officers are still exempt. One last note, only sworn peace officers are allowed to carry firearms from out of state, any other position, civilian or non-sworn, within out of state law enforcement agencies are not afforded this right. We have also run into a variety of other situations where a state may allow a specific position within an agency to carry a firearm, but under the international agreement, only peace officers are allowed to carry firearms across state lines. If you have any additional questions, please let me know. Thank you.
CHP Customer Service