All right, time for me to wade into the deep and conflicted field of survival firearms. I'm not going to get into the age-old arguments of 9mm vs. .45 ACP, AR-15 vs. AK-47, etc., but instead offer a somewhat different viewpoint on where to start/focus your firearm preps.
Experts often focus their recommendations and thoughts about big battle rifles--.308s mostly, like the FAL, M1A/M14s, etc.-- or lighter battle carbines, like the AK-47 or AR-15. I'm not going to argue for a second that the a good rifle is the weapon to have in a gunfight. They've got the power, range, and accuracy. They'll punch through cover (especially battle rifles), and they'll take out what they hit.
However, most often, a good ol' handgun is the more useful and practical weapon to have.
The best gun is the gun you have at hand when you need it, and more often than not, that will be a handgun.
Despite the frequent fantasies of gunning down hordes of mutant zombies, we're in this to protect ourselves and the lives of our family, not become John Rambo. You don't need to become the ultimate soldier to survive, and in some cases, the uber-tactical gear might just get you singled out by the mutant zombies/mobs/authorities. The guy carrying the long gun will always get more attention than the guy who appears to be unarmed.
Attention is bad. Going unnoticed is good. Avoiding unnecessary trouble is key.
For going unnoticed, the handgun is king. It can be easily concealed or carried low-profile.
Rifles tend to be big, unwieldy, and heavy. And then you've got the mags and ammo, an extra burden. No big deal if you're holding up in your house, but if you're carrying your gear, it can add up. If you're out and about, driving your car, working in the yard, or whatever, you're likely to set the rifle down. Even when slung, it'll get in the way fast.
The handgun, you have holstered at all times, always ready. Driving around, working in the yard, carrying water from the river, etc., there it is, at your hip. The handgun is gun that you have on hand, always.
Despite our wild post-apocalypse dreams, the fact is that society will not instantly collapse during most disasters. The situation may be short term, or only enough to lead to hard, dangerous, and troubled times for all. You may well still need to (or be able to, depending on how you look at it) go to work, go shopping, or otherwise operate in society. Things would have to get pretty bad for it to be socially acceptable to carry an AR-15 to work, but a concealed handgun is another story. We want to prepare for the most likely first, and to me, a partial collapse, with a drastic rise in crime seems much more likely than total collapse and anarchy.
Realistic, practical, low-profile, always ready...that's the handgun, and those should be the attributes we strive for in our emergency/disaster/end-of-the-world preps.
Rifles hold a special and an essential place in preparing bad times ahead, but if you're only going to have one gun, make it a handgun.