AFAIK, LEO's are exempt from the firearms restrictions mere citizens are subject to.
There are California Legal AR type rifles available for sale, but they are configured to pass every restriction the state has setup. As an example, here is Colt's web site showing their selection of compliant rifles:
NRA Chief Range Safety Officer, Home Firearm Safety and Pistol Instructor
"There are three classes of people: those who see, those who see when they are shown, those who do not see."
Leonardo da Vinci