A Ma Deuce fits perfectly in the bed of a '74 Datsun pickup. However, CHP will cite you if you leave brass on freeways.
What if a person doesn't own a pickup?
Where's the article about chic guns for Bentleys? I believe that Rolls Royce upgraded package includes a portrait of the queen and a Springfield Armory Professional Model .45 ACP. Muscle cars have all the defense they need under their hoods. A couple years ago, I saw a new Shelby Mustang accelerate on the S/B I-5. That beast accelerated out of a pack of typical automaton So Cal drivers in a heartbeat.
Here's the reality:unless you're a rancher, a long gun is unwieldy inside of any vehicle. The probability of a long gun shootout from a vehicle is less than nil. If a long range shootout were a possibility, I'd make it a longer range shootout by putting miles between me and an attacker.
If I were to have a vehicle-dedicated handgun, it would be my Springfield Armory TRP .45 ACP.
While I'm deep in the Rockies, and I'm in the Rockies a lot, I never carry a long gun unless I hold a big game tag. When I'm deep in the Rockies where mean critters roam, I'll have my TRP close by, even closer if my kids are with me. Ain't no mean critter gonna touch my kids. A dad's dominant responsibility is to protect his kids.
If I were a rancher and had to protect stock from predators, I'd go with a scoped .308 Win carbine, probably an accurate AR version.