Bet-your-life reliability in a 6.5 Grendel AR15 platform?

[Sticks]

.... As just one example, I live in Pittsburgh where we have long, traffic laden tunnels. It is not unthinkable that some whacko could use a traffic situation like that to create an ambush from which I could not escape and would want more than a compact handgun to deal with.... .

You actually have a better chance in a tunnel full of cars than in the open.

Same theory applies. Keep your head. All manner of cover and concealment in that situation. Exits are limited, but hard cover to wait the incident out are plentiful. If the shooter starts walking up the line of cars shooting, then you are right back to your 9mm being the weapon of choice to defend yourself if the need arises. Hard to point and aim a AR pistol from under a car. Little on the tall side.