I keep a Mossberg 590 and a Sig 556 SBR in my truck at all times along with 6 mags of 5.56 and 20 12Gauge slugs.
not.....It never fails to amaze me the number of people that keep "heavier" firepower in their vehicle and not some of the other basic things needed in a firefight, I carry a pistol as a deterrent and if absolutely necessary a means to end someone elses life before they end mine. Occasionally I leave my 835 in the truck with some turkey shot, more for a spur of the moment hunt at dusk or on the way to work. If I encounter a situation where I am out gunned and looking for rifle, they probably have a rifle as well which means I want some kind of SAPI's or substantial cover neither of which I carry with me on the regular, sure I do have a BOB, but thats more of a responders bag, mostly my kit bag for EMT duties with a spare mag for my carry gun, should I get a call and be at home that involves shots fired? Yes, I'll grab a flak and shotgun, but lets face it. I'm not a SWAT member, I'm not a National Guardsmen looking for a silver star, I carry to protect myself and those around me, if confronted by an armed gunmen with a rifle, best bet is to keep covered and treat casualties, and pull back to regroup, if I can get a shot to take him out with a handgun, it will be done but I'm not going to run around with a combat load and vest hoping to save the world. Now for the OP's question, leaving a pistol in a vehicle isn't a great idea, but I have done it from time to time, usually locked in a lockbox under the seat or in the glove box.