This concept has been kicked around on internet boards and in firearms circles for some time. The concept is never really defined. As near as I can tell, the term refers to a firearm that can be kept in a vehicle for an emergency. One of the criterion of truck guns is that they are so low cost that their loss or theft won’t be a problem for the owner. So the #1 criterion for a truck guns is that “I don’t care if someone steals it”.
Of course this is unrealistic. There are no inexpensive guns any more, even the tough old Mosin Nagant rifles cost about $100 and ammo adds more cost. Other guns are much more expensive.
It ignores the problem of a stolen firearm falling into wrong hands. It also encourages confronting a situation with “less gun” that would normally be desirable.
So what is a “truck gun”? Well it should be the hand gun you concealed carry. It should be the rifle that can protect the best. It can be a plinker. Like every other firearm it should be matched to its intended use.
For my part, my car or Jeep gun is an Israeli surplus ArmaLite AR-7. Purchased about 15 years ago when a large number were imported. Their price was modest and the quality is high. I don’t think it’s a great defense gun, its not. Nor is it a great hunting rifle, but, it’s better than not gun at all.