[Weapons decision matrix in a survival skills thread]
In military branch planning we start with optimal expected conditions, and then take away something, and re-plan; do it again, and re-plan ... keep going until you reach the point at which you have no reasonable chance of success (barring divine intervention). Then go back up a step, and ensure you are manned, trained and equipped for that level. Then go up a level, and do the same, and so on, and so forth. My humble opinion is that many preppers/survivalists start with the best case scenario and prep away, with maybe a nod to a few bad contingencies. This is the pallet of FMJ, and a box of parts and tools with a manual approach ... that's great if things don't get as bad as they could. My decision matrix starts with the worst case scenario ... on foot, nomadic, for an indefinite period of time (think "The Road"), and I have manned, trained and equipped to that level first and primarily, then added some nice to haves.
For that a firearm needs to survive 3+ generations of regular use, with no expectation of parts, tools, or skill to maintain. Must be operable by the small, sick, elderly, young, or injured ... one hand, weak side possibly. Before anyone says they do not exist, I have an 1897 .38 S&W revolver made by Iver Johnson that will put all 5 in a pie plate at 10 paces, and has been safely handled for 4 generations of people aged 4 and up. (No, that's not what I'll be taking, but if its all I had on hand, I'd not use it on myself in despair.)
So for firearms, I like quality revolvers, in a fully enclosed durable holster riding on a durable belt around my waist. Never have to set it down, always within reach, and keeps my hands free for other things. I shoot pistol competition, and take my family pistol shooting primarily in preparation. (All of them have rifles and are proficient in their use as well.)
For calibers, I like .22 and .38/.357. .22 because they're plentiful and effective, and .38/357 because they're easy to cast/reload for, can be loaded with the holy black in a pinch, and a good cast bullet will fell most anything at appropriate ranges with a well placed shot, without causing a flinch in new or inexperienced shooters.
My plan is a .22 in the pack, and a 357 on the hip, for everyone in my party, bare minimum. To that we've also added some other handguns, long guns, and shot guns, a few of which we'll carry as long as we can, but won't cry if we need to drop them to surmount and obstacle for survival. They'll also be handy as long as we're able to stay home or in a fixed location.
Things like range and energy can be compensated for by tactics, skills and practice. Average shot on a deer in the US in 2011 was like 2009 ... 80yds. Doesn't matter if it was a 300WSM, 308, 30-30, or 357; at 80 yds the deer don't know the difference. And if we're in a worst case scenario, deer will be rare ... cats and other rodents will be a delicacy. I know most people worry about engaging well armed gangs bent on rape and destruction, and I do not doubt that's a possibility. There are other ways around that too, that don't involve wife and children in SAAPI carriers, toting an M4 with beta mags ... I suspect.