For centuries, firearms have served as society's iconic weapon for self-defense. Nothing quite screams "don't mess with me" better than a big gun, right? But what role does a firearm realistically play in a disaster scenario? This could easily transition into many of the "great debates" of firearms, but for this discussion we're just going to focus on the philosophies of use, as well as realistic solutions. This will cover firearm use strictly from a self-defense perspective, so please note that hunting is a completely different topic not considered in this discussion.
In movies and TV shows, nomadic heroes manage to survive the most grueling, harshest environments carrying only a machine gun and a bandanna. As I hope many understand, this is Hollywood distortion at its best. Should a disaster scenario occur, a firearm is not likely a main priority such as food, water, shelter, and first aid -- bullets cannot keep you nourished and hydrated. But after such provisions have been situated, then protection of loved ones and supplies is often the next logical step.
Before diving in, it is important to note that conflict aversion should always be the path chosen. You can never predict the intentions/actions of another person, particularly someone who may be in desperation, so don't go looking for trouble. But when cornered, firearms serve their role in your system. This means that your firearm should be considered as a defensive tool -- there is a much bigger difference between being "unarmed vs. armed" as opposed to "pistol vs. rifle". Having any firearm puts you in an advantageous position, or at the very least brings you closer to being equally-matched.
CHOOSING THE RIGHT ONE
Cutting to the chase, a mid-to-full sized pistol would be my choice if I chose to own/bring just one firearm for self-defense. A larger pistol can typically still be concealed, yet often has better shootability, capacity, and recoil management than their compact-sized counterparts (at least for most shooters, some claim to shoot compacts more accurately).
Regarding caliber choice, take your pick -- because no pistol is a rife or a shotgun. As most have agreed, it is wise to choose a commonly found caliber, but most avoid .22's for the fact that although still lethal, they are generally the least effective of common calibers and not considered to be quite as reliable as centerfire ammunition (subjectively). Although pistol calibers/types do perform differently, statistical data is second to your confidence and ability to use your weapon safely and proficiently. Get yourself some ears, eyes, and a range bag and get to the range for practice! Choose what you feel suits your needs; firearm choice is often a personal formula to which an answer exists in the vast world of guns available today. If you absolutely do not know where to start, a good 9mm pistol leaves little room for regrets. The practice ammo is inexpensive, it exhibits milder recoil (and therefore better user-accuracy) and wear, possess a higher capacity than most other viable pistol calibers, and is absolutely considered an effective defensive round. Don't get too caught up in pistol caliber ballistic data wars. No sane person is going to look down a barrel and say, "Oh thank goodness it's only a 9mm". Also bear in mind that any firearm is capable of defending you without firing it -- its presence alone can change the outcome of a conflict. And if that doesn't suffice, a warning shot will likely be enough to send the message that you're armed and cause the threat to think twice before pursuing.
No matter what you believe to be the best firearms/calibers, it must be reliable. While most recognized manufacturers now build reliable firearms, there are still many substandard manufacturers that are best avoided. As of 2017, it seems to be around the $400-$550 area that brings you into bet-your-life reliable classes of pistols. While more expensive pistols exist, they tend to cost more from refinements in other areas while reliability/durability will be essentially the same, especially for average user. Not to say the refinements aren't worth the money, but you don't need to spend $1,000 to get a reliable firearm. Everyone has their favorite manufacturers/designs, and most are viable solutions. So to keep the peace, let's not travel down the road of "best overall pistol".
Lastly it should be noted that hollowpoint bullets are ideal for self defense. There are two primary reasons for using them: 1. They expand upon impact creating a more severe cavity wound, and 2. The expansion utilizes the energy transfer more efficiently upon impact, reducing the chance of the bullet passing through to an innocent bystander (aka "over-penetration). While hollowpoint bullets are significantly more expensive than ball/full-metal-jacket ammo, they are well worth the investment. While you are not likely going to practice with such expensive ammo, it is always recommended to put a sufficient amount of the hollowpoints you plan to carry through your firearm to ensure it functions properly in your specific firearm(s). No need to wait for a life or death situation before you find out your pistol and your new ammo don't play nice together.
PURPOSE OF USE
When discussing potential emergency situations, it is generally broken down into three basic categories: stay home, get home, or leave home.
For the "leave home" and "get home" situations, we're going to assume you're on foot and limited to what you can carry, hence my suggestion for a pistol as it is going to be the lightest and most concealable. In terms of mobility, you'll be weighed down by the non-firearm related gear alone. So in my opinion, no more than 5-6 lbs should be carried for your pistol system. That's enough for a 1.5-2.5 lb pistol and a modest amount of ammunition/magazines. You should have enough ammunition to survive a couple small encounters, but you'll never be able to carry enough ammunition to single-handedly win a war. So carry a reasonable amount of rounds and keep your mobility as the priority.
Obviously there are a multitude of situations where a more suitable firearm exists. I'm sure many may prefer a rifle or shotgun, but when foot, I believe there is a need for both weight reduction and concealability. The theory behind this whole topic is of course subjective because putting a firearm on display may serve as a deterrent for hostiles, but may also inadvertently challenge someone or label you as well-prepared (and therefore a target for supplies). I would prefer to travel inconspicuously, therefore my need for concealability exists. Assuming the emergency has not caused a total societal collapse, there will likely be local authorities patrolling who may not take kindly to those openly carrying a rifle in plain sight. This could result in your firearm being seized/confiscated, leaving you unarmed as opposed to if you had just concealed it to begin with. There are some great modular packs, holsters, and carrying systems out there designed for concealed weapons carry. Experiment and find what works best for you.
Now for "stay home" situations, this gives you much more flexibility. While a reliable pistol will still serve you well for home defense, the door opens much wider when you're not limited by size and weight constraints. An AR/AK-type rifle, 12g shotgun, .22 rifle, and scoped bolt rifle are just some of many great additions to your home armory for various purposes and geographic locations. It's up to you to decide which firearms are most advantageous for you personally, but having some versatility and redundancy is always a good idea. If the situation is of severity that forces you to scavenge, you'll be happy to have firearms that use common calibers so you have a higher chance of being able to utilize any ammunition you may find. A good example would be a .357 magnum revolver also capable of shooting .38 special and .38 special +P. Another perk of being "hunkered down" at home is the ability to store as much ammo as you can fit/afford! Although check with local authorities about storage limits on ammo as there may be restrictions.
In addition, a rail-mounted flashlight or flashlight/laser unit is a good investment for any situation. You'll need to identify a target prior to engaging in low light conditions, and although a laser can never replace your sights/optics, there may be a situation that prevents you from aiming with your sights. At the very least, keep a good flashlight near your firearm.
While we are on the topic of purpose of use, note that a firearm is not always the best tool for self defense. The two major fundamental flaws of firearms are: they are overwhelming loud, and essentially finite (bullets). A gunshot in just about any caliber is devastatingly loud without hearing protection, particularly indoors. This will not only alert everyone within miles of your presence, but can cause severe disorientation and potentially permanent hearing damage/loss. There are plenty of videos and articles of people having an accidental discharge indoors without hearing protection, consequently hindering his/her hearing for a few days at minimum. Yes, certain calibers are louder than others, but rest assured that you will definitely hear it! This is another Hollywood misrepresentation: people shooting automatic rifles in an indoor gunfight just to have a friendly casual conversation afterwards -- not likely because your ears would ring like sirens! So then why don't we just plan to use hearing protection? Mostly because you may not have time to properly protect your ears before reacting in a threatening situation, but also because hearing is one of your best tools for awareness in your environment. A solution to this may be a suppressor, but many states/countries do not allow them entirely. Clearly there's some paradoxes here, but just be sure a firearm is not your only tool for self defense. A solid knife, machete, or axe serves as quiet, close-quarters defense tool, and are also capable of nearly endless uses other than self-defense.
Firearms are like any other moving machine: it wears. The life and performance of a firearm is heavily dependant on the owner's treatment and care. While you may see several pieces of literature stating a particular firearm's "service life", these are figures represented under the assumption that minimal, if any, routine maintenence is performed. Assuming you properly maintain them, most firearms will last several persons' lifetimes when fired on an average volume to far exceed round counts for their stated service lives.
Regardless of the firearm(s) you choose, it's worth briefly mentioning that additional magazines, cleaning products, and spare parts/springs are worth investing into as you may not have easy access to these things in emergency situations. I'd also suggest keeping a range bag readily available that contains all the ammunition and parts you would want to take with you should you need to leave the premises. In addition, you should pack a very small amount of your favorite oil/CLP/grease with you to keep your firearms running reliability. The same products can also generally be used to clean/lubricate/maintain other metal tools you may have. Much of preparation is simply proper organization while you have the luxury of time to prepare.
There is no perfect firearm for everyone in every situation. There is an overwhelming plethora of information on firearms online to make a good choice, but I cannot emphasize enough to first spend a day renting various firearms to see what works best for you. Sometimes you just shoot certain guns better with no scientific explanation, but you just do! Once you've found what you like to shoot, do the research afterwards to either solidify or nullify your decision. Often people find a particular platform/system (i.e. sticking with one or two manufacturers for proficiency/familiarity) and build upon that. Others may want as much variety as possible, while another may just want a single reliable firearm and a few spare parts to compliment it. Love 'em or hate 'em, firearms exist, and everyone at minimum should be familiar in their operation and safety.
If there's one big thing to take away from this it's: firearms MAY save your life, but food, water, shelter, and first aid WILL save your life. While a firearm is important, do not neglect other categories that are more likely to cause concern. Firearms should be just one piece of your whole system to prepare for the unexpected. Stay safe and stay smart!
***Please consult your local and state laws regarding the ownership and carrying of firearms.***