No regulation as to what kind of firearm is carried in a vehicle.
I think he was asking about the firearm being loaded. I might be mistaken, but IIRC, a pistol can have one in the chamber but a long gun can not. Hence the reason some are now carrying the AR/AK pistols.
As for the original statement by CM- Several years ago I too decided to change my truck/trunk gun to something that will not cause me big tears if stolen and will put it in the hands of a thug. I went to a 12 Gauge pump with loads in slug and 00 or #4 buck. We all must make our choices...
Oh- Happy Thanksgiving all! JP