A lot of folks have different ideas and needs in a "truck gun". Some like handguns, some shotguns, and some rifles.
I used to keep an 8" .357 mag that I called my pocket rifle in my glove compartment in the big honkin jeep but then I realized I seldom leave without a side arm and had a whole truck to carry in and could use whatever I wanted. I have a nice little model 11 cut down into a carbine that's great for things that need shooting while out kicking around and fits well in the truck.
So what if anything are you guys using as a vehicle carry?