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.
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Q: Why carry a handgun? Are you expecting trouble?
A: If I were expecting trouble, I'd carry a rifle.
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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.
