A good truck gun is one that you don't have a lot invested in. Like say you wound up with a handgun someone threw in on a trade to sweeten the deal. That would be your truck gun. Pretty much any cheap snub like older Rossi or Charter Arms stuff. Makarovs PA 63s,most Ruger P series pistols etc. As one of the earlier comments stated, if you have under $300.00 in it then it certainly qualifies as truck gun material. This category USED to include any SKS and some AKs. Way back when it would've included M1 Carbines. They were perfect truck guns especially the paratroopers. But these days the afore mentioned rifles command too much money to fit in this category. But yes, Hi Points and Jimenez pistols certainly fit in this category. I don't have a truck at the moment but I DO have an old RG snub in 32 Long that I'm really attached to.
I EDC and no longer leave a gun in the truck. There once was a time when "truck gun" brought to mind a used weapon someone carried for general use such as dispatching snakes or wild dogs out on the farm or at the pond fishing. I think its used now just like the "handy man special"...in other words its an old piece of crap that I dont have a use for but its good enough for you to through under your truck seat, if you even have truck.
A while back some ass hat hit my trunk in a parking lot and drove off, I had been inside shopping for an hour or so and came out to the parking lot to see my cars trunk wide open. It closed back fine, but if I had a rifle/shotgun mounted on the trunk like I originally wanted it would have either been stolen or cops would be all over my car with idiots freaking out cause oh no it's a gun!