At least here in NC, some departments do either issue them to each officer, or as a pool "rifle" to a group of officers. Those that agencies that dont issue them (and some that do also) allow personally owned rifles to be carried in the vehicle, once the officer qualifies with the rifle.
There is still a rifle loan program for law enforcement agencies to get military m16's of various vintage. Unsure what is currently available though. Last few I saw were M16A1's.