There's been a lot of discussion and some disagreement at Calguns over this. What's not in dispute is that LE, with a department letter, may own an un-neutered AR15, or "assault weapon" as they are defined in CA law.
Citizens in CA may own any AR15 pattern rifle that isn't named on the AWB list, as long as it is either "featureless" (no pistol grip, flash hider, etc. see flowchart
) or that has a bullet button (a mag release that requires a tool to operate it).
No magazines greater than 10 rounds may be used in a bullet-button AR if the rifle has "features" without creating an assault weapon under CA law.
Former AG, now governor Jerry Brown, once opined that the law, as written does NOT allow LE to retain their rifles after retirement, however previous 'opinions' led officers to believe they could keep them, and a number of weapons were purchased with that promise and understanding.
There is actually legislation
that was introduced recently that would, among other things, limit the number of so-called 'assault weapons' (including 50 BMGs) that an individual officer could purchase under the LE exemption to a total of one.
Apparently there are public safety risks when officers have an 'assault weapon' in both their right and left hands.