[danso]

Police officers' main roles, ideally, are not to display a show of force. Think about neighborhood beat walks, to writing tickets to motorists, to handing out citations and doing stop and frisks. In all of those situations, handling a rifle is extremely cumbersome never mind socially awkward. FWIW, in some/manyjurisdictions, patrol vehicles come equipped with a shotgun.

[fivestar]

Based upon the huge number of surplus police shotguns presently for sale, I suspect that most patrol cars now have rifles in them.

[azernik]

Indeed they do. Joke I've heard:

" Q: Why carry a handgun? Are you expecting trouble?

A: If I were expecting trouble, I'd carry a rifle. "

i.e., the pistol is an everyday-carry weapon, to take with you if you think a situation might be resolved without use of force. If you know going in you're going to get in a gunfight, you go to the car and grab your shotgun or rifle.