I know you specified a long gun, but I'm going to go against the grain here and suggest something kinda oddball - a FiveseveN.
Now hear me out - it's compact enough to stow in the glovebox or center console, and the round shoots flat enough to reach out and lay down suppressive fire at 100yds or more. Further, with the right ammo, it will penetrate car bodies with ease and retain enough velocity to get an attacker's attention. It also has a large ammo capacity and is ammunition is lightweight.
I currently use a CZ-52 in this role, due to the penetration of the 7.62x25 and the confines of my Honda Accord. If I had a FiveseveN, that's what I would have filling that role.