If you want a gun in case social order breaks down, Geek with a .45 is suggesting the Keltec 2000. This is one of the pistol caliber carbines I mentioned in passing here. It costs around $300 and models are made that accept various pistol calibers and magazines. It also folds up for easy storage.
UPDATE: In other SHTF preparation news, Instapundit is linking to this University of Wisconsin water storage advice PDF. One of the important points is that old milk jugs are bad places to store water. Two liter soda bottles are much better. This is because milk jugs almost always contain bacteria that can't be removed from simple household cleaning. Soda bottles don't have this problem.
I can say that this is true from personal experience. As some readers know, one of my weirder hobbies is swordsmanship. One of the most common targets used for cutting practice is old soda and milk bottles filled with water. I built up quite a stash of these over the colder months when you don't want to be outside swinging a sword.
When the weather warmed up, I started using them. The soda bottles were all fine. The milk bottles were not. They were completely disgusting. These were empty bottles that had been rinsed clean, screwed shut, and stored together in the corner of my apartment. It may take a while for the milk jugs to get nasty, but they will. Don't store water you intend to use for hygenic purposes in them. If you need water to keep the toilet flushing, so be it.