12 gauge !!!
* cheap
* robust
* comes in many, many formats (slugs, SSG, buck shots, tracers for signaling, you name it...)
* can be found just about anywhere in the world
* even if the barrel is a little dirty, it won't explode in your face as easily as a rifle.
Actually, a short pump action is my #1 pick for just about everything, from hunting to survival and urban self-defense. In short range, my guess is that slugs can even disable a guy wearing kevlar... the impact is just tremendous...
I've killed a bear with a slug, once. Hit him in the face. It took most of his head away...
The only problem with 12 gauge is the limited efficiency at a longer distance...
Cheers,
David