I've been bored on post for long enough to start giving situations, and having them prepare for them. Fun way to do tactics.
I also have my own personal plans.
I've recently fallen in love with the Rossi Circuit Judge.
http://www.rossiusa.com/product-list.cfm?category=15 Carbine that shoots both 410 shot and .45. I just get an orgasmic feel from it. Heard good things too.
A .45 would be accurite out to about 100 yards in it, which is good for a pistol round. It's fairly small, and you can switch ammo with that silenced .45 pistol that was mentioned. And can also load small shotgun loads. Yes, it's 410. Smaller than 12 guage. But it's also lighter. And at short ranges, does just as good a job.
Anyway... I just really like the gun, and want one really bad.
Other guns would be dependant on the situation. If there is a military presence, then everything hits the fan, pick up an M4 off the ground. 5.56 will be more than enough with an accurate shot. And ammo will be easy to find.
Kel-Tec makes carbines that can accept Glock 9mm magazines, and 9mm is almost the most common ammunition you can find. And it is enough to do the job.
My plan would be to remain as mobile as possible. Only a short term solution, but enough to let the starting chaos die down.