Cheaper Than Dirt has a great post about which rifle to keep in your car:

To understand where the concept of a trunk gun came from, let’s go back a few years. Traditionally, farmers and ranchers often had a ranch rifle hung in the back of their pickup truck. The role of the rifle was to enable the easy elimination of varmints, or to be used to put down injured horses or livestock. Nowadays, many farmers and ranchers still keep a durable rifle hung in the window or stashed behind the seat for the same reasons. But in addition to the traditional reasons for toting along a truck or trunk gun, many people in urban and suburban environments see the usefulness of having a rifle somewhat readily available in the trunk of a car for self defense and general preparedness. For preparedness minded individuals, the trunk gun is often paired up with a BOB (Bug Out Bag).

The article mentions several rifles, including my beloved Kel-Tec SUB-2000. Check it out here.