You mean a rifle under the seat along with the usual handgun in the glovebox?
Depends on probable likely employment, local conditions, and local customs and familiarity.
For an indoors office worker, the answer might better be a handgun on his belt while indoors at work rather than out in his vehicle. For a rural resident, a handgun might be near-worthless at the ranges likely to be encountered, and the scoped hunting rifle he uses to harvest an annual deer for the freezer would be a much wiser pick.
In my own case, in the urban Memphis area, a variety of situations are conceivable, and incidents ranging from highway sniper attacks to an attempt to hijack a cargo jet and crash it into an occupied structure have taken place in the past, so are certainly worth consideration for forestalling as potential repeat terrorist attacks.
Rather than *under* the seat of my truck, my accurate rifle resides in a fitted case behind the folding seat of my pickup, as required by state law, though I can be exempted from that regulation in the time it takes for one telephone call's notification. But nevertheless, I'm very comfortable with the equipment that's my usual daily gear, and have augmented my usual kit in only a couple of ways.
There's a little more ammunition and of a couple of additional types than usual along with me now. And I'm carrying a shotgun along as a spare, should I be detailed to work a roadblock checkpoint, as has happened in the past, or should I have to better equip a partner with only a handgun, who's not had an opportunity to pick up more serious hardware of their own.
I'm more likely to be carrying an M1911 model military .45 or my 9mm Browning GP than the little 9mm Makarov handgun I'll sometimes resort to for Summer wear, and it doesn't hurt matters any that I also have three 30-round magazines for the Browning that work flawlessly, as well as a pair of 17-round reloads on my duty belt. And if 9mm jacketed M882 *hardball* is sometimes thought of as less than a terribly effective load, it does penetrate automotive sheet metal and glass handily, and is utterly reliable from my GP.
But since our state law doesn't allow a loaded rifle or shotgun in the passenger compartment of a vehicle even if the operator has a concealed handgun permit, I long ago came up with an *imitation rifle* to both meet my needs and stay within the letter of the law. In this case, the *rule-beater* is a C96 Model Mauser *broomhandle* pistol of the type once used by Winston Churchill in the latter days of the Nineteenth Century, no less, though somewhat different from the configuration his was in. Mine has been fitted with a 20-round magazine extension, both offering the opportunity to miss a lot if I'm so inclined, or to reload with a clip of 10 rounds of Czech 7,62x25mm machinepistol ammunition or my equivalent handloads with the *pistol* half-emptied.
Too, the 100-meter adjustable sights are a wee bit overoptimistic, but silhouette targets at 200 meters are pretty easy, and even at 300 are quite possible. That the *pistol* is equipped with a detatachable stock that doubles as a holster certainly helps, but despite that configuration, it technically and legally remains a *handgun* and so has served as my *vehicular carbine substitute* for the last several years here.
And whether the legalities apply or not, I'm used to it, and it'll do. For three centuries now the old Mauser design has served its purpose and done its job, and for me, it still does...along with the others.
BTW, isn't a shoulder stock on a pistol a no-no?