Guns are only part of it. And the choice of actual weapon is not so important as insuring resupply of ammo and to me that means military calibers - .308, .223 and 9mm. If the SHTF, 7mm-08 and .270, even .30-06 and .357 Magnum ammo, will quickly be in short supply or - poetically likely - stockpiled in huge quantities in the hidden basement vaults of guys who starved to death or died of an infection from a minor cut.

Amateurs talk tactics, professionals talk logistics. Being able to defend yourself is certainly important, but as already mentioned, food, potable water and medicine would have higher priorities than the ability to repel boarders.

Here is a decent book on the subject:
http://www.amazon.com/Crisis-Prepar...mp;s=books&qid=1263852838&sr=8-1



Gunnery, gunnery, gunnery.
Hit the target, all else is twaddle!