Some good discussion on bringing More Gun in the car just in case. Various intermediate rifle caliber pistols were brought up, and many of the comments went straight to "NONE OF THOSE, if you have those the police will shoot you on sight because badguys also have them!"
This isn't quite accurate. According to a lot of stuff I've seen on training exercises, under the stress of even a simulated active shooter situation, police tend to go straight to "If gun, shoot". I'll find the specific articles if I can, but the gist of it was a simulated active shooter response, with unknown numbers of police responding from different angles. Police were shooting other uniformed police, because they were shooting immediately upon seeing the gun before anything else processed, and most of the blue team casualties in the exercise were friendly fire. Just about the only person that didn't get shot on reaction? The refs, wearing orange safety vests, the mark of "Do Not Shoot" in almost all paintball, airsoft, training exercises, and just about anywhere else that's got living no-shoots.
Further proof for carrying effective things even if they're Evil Black Rifles; Most police have them, under the term Patrol Rifles. I'd be astonished if there were no police in all the country who didn't carry AK's as their patrol rifles instead of AR-15s, in some region where the officers have to provide their own.
If you're going to the trouble to have a truck gun, you might as well throw in a safety vest with it (camouflage in urban environments is nearly useless anyway).
If you go for a rifle pistol, I'd suggest putting a bipod on it in case you need to shoot at long ranges where pistol kinematics make sufficient precision hard.