squad car implies (to me) urban, semi-urban, in-town patrol
leave the centerfire rifles and 100 yard shots to the SWAT guys
that is what the radio is for
(same for ARs, AKs, M-16s, M-14s, UZIs, "machine guns", and grenade launchers)
every squad car oughta' have both a pistol caliber carbine (16" to 18" barrel w/ shoulder stock, of course) and any one of the usual 12 gauge pumpers, both secured but readily available
what you carry when leaving the squad car being situational
nothing beats the intimidation factor of the 12 ga up close and personal, but carbine is far more shootable & effective for less close and well aimed
I would go with 357 mag lever action carbine (w/ 357), 12 gauge, companion 357 revolver (w/ 38+P)
but "the dept" will likely insist on a semi-auto pistol, so match carbine caliber to handgun caliber in any/all cases