Shotgun, pistol, ammo for both.
I like the way think Cedfire.
Actually, this scenario came out of my head because I have an ill prepared daughter and her boyfriend living in Portland Oregon. And you are right, you don't necessarily need to bug-out from volcanic ash drop....it just fit my scenario. It could have been a tsunami heading up the Columbia of a dirty bomb somewhere.
I would be carrying a 12 gauge 870 with double 00 shot in a quickly accessible soft guitar case, and a .22 Sig Trailside in the BOB. The .22 ammo would be for food and the shotgun for both protection and food. It's tempting to want to take a .45 too but the weight of the ammo is 4 times that of .22 ammo and a shotgun is a very good self defense weapon.
So let's take this scenario a step further. You and your partner have left your car and you are loaded with emergency preparedness equipment in an unfamiliar urban area that turns out to be, "not the best side of town". There are thousand of people trying to get places, in vehicles, on bikes and by foot. How do you proceed on foot through an area where there may be potential looting and/or theft? Do you travel back streets and alleys or stick to the main streets? What would you do to fit in or to become a chameleon? (I think I just talked myself into bringing my .45 too.)