Been carrying a 12 gauge shotgun in my vehicle since Reagan was Prez. Most all LEOs in my hometown as well. You can carry them across nearly every state line and I've taken one thru Mexican and Canadian customs.
My reasons...
Learned more about weapons serving my Uncle and got to shoot pretty much every caliber out there and carry them(the smaller ones) in vehicles.
Never got tapped while driving in bad guy land but several friends did and dismounting and shooting takes some thought and coordination for future thought.
Shotguns are crowd pleasers, not to take anything away from handguns and ARs.
However...
Lead dog in the stack in house takedowns in Iraq nearly always had a Moss burg 500 handy. Having been issued a CAR15, M9 and 1911 in my past I see all your points with good reason and measure.
For me: give me 00 Buckshot inside 20ft any day...
A police style shotgun mount for my Raptor is in my future...
V/R
Smokey
Great thread!