It depends on the agency
Law enforcement agencies are pretty decentralized when it comes to firearms, with each making choices based on cost, or even the personal preferences of the firearms instructors who usually make the purchasing recommendations.
I doubt you'd find any agency in California still using Winchesters, M-1 Carbines or tommy guns anymore (or Reisings, for that matter), although supposedly in some rural departments, the officers purchase their own weapons. There is a provision in the state assault weapon ban exempting peace officers, presumably for this reason (they are also exempt from high capacity magazine bans). This is abused, IMHO, with some officers purchasing items for personal use that are otherwise banned.
It's interesting to see what rifles different agencies show up with. CHP and LAPD, I think, get surplus rifles from the feds, and I see M16A1s and -A2s being carried, instead of the more usual carbines. They might have a locking plate installed under the pistol grip to preclude full auto or burst firing, but I don't know.
HK SMGs of various calibers and types are often carried by special weapons teams. Fish & Game (oops, Fish & "Wildlife"...) carries M-14s, as does another small local agency hereabouts. I've also seen Steyr AUGs with at least one agency, but that was some years ago.
I've seen some deputies with short-barreled pump shotguns, which require a Class III stamp like a fully-automatic weapon.
My agency issues Colt 6920s, a semi-auto AR carbine, which is slowly supplanting the Remington 870s. We don't have enough rifles for every officer, but I'm not buying more shotguns because I anticipate that eventually we will transition to using them for less-lethal force options ("bean bag" type rounds). I, myself, usually carry a shotgun because the vehicle I'm driving now only has a shotgun rack.
I think at this point in time, semi-auto M-4 type carbines are all anybody is actually buying. However, agencies will continue to deploy whatever they have in the rack, especially for these big deployments.
And, yes, 30-round mags are standard, in spite of the state ban on mags exceeding 10 rounds...
Last edited by R1145; February 11, 2013 at 11:22 PM.