Regarding Post 20.
I only fully retired 5 years ago. In my California department where I worked, it's my understanding the Calif. academies generally taught the pump-action shotgun and not a carbine, but I don't know if it was a Calif. requirement or just standard operating procedure. I also don't know if the larger departments, which might be able to conduct their own in-house academies, could leave out the shotgun training; or substitute-in rifle training.
Where I worked, rifle training was individually-requested and approved-issued case-by-case/as-needed/time with time & budget considered. Perhaps the OP has more knowledge as to how Calif. police academies and Calif. "POST" operates. The shotgun was more typically a "pool" gun with only a few actually issued to an individual whereas the AR15 was issued to an individual. My department didn't allow personally owned handguns (except for BUGs), shotguns, or rifles. Some other departments would allow more personally-owned-duty firearms, although prior to me being hired there, it used to allow such. My prior department allowed personally-owned firearms, but this was prior to rifles/carbines taking off like they did.