I generally don't carry a long gun in my vehicle, though I have been mulling over getting a Kel-Tec subbie as a truck gun. If I had to say what's my "go-to rifle" it'd probably be my CZ 527 in .223 with a nikon 6-18X40 on it. Only because it's usually what's in my truck if I have a rifle in there, and if I grabbed a rifle at home to take care of a BG past 50yds that's probably what I'd grab. If they were out past 300yds I'd grab the old A-bolt in .338 topped with a Zeiss 4.5-14.
I'd pick the subbie as a truck gun due to it's size, simplicity, ability to fold, and most of all...they're pretty cheap. If I'm gonna have a truck gun I don't want it to be one that I'd be too upset about getting dirty/rusty or stolen.