I have tried several times to pick out one rifle or firearm that would be my go-to gat in a SHTF or Bugout situation. I've even posted several replies on the forum with my opinions in a long, drawn out way explaining why this or why that. (warning: this post will be no different) When it comes down to it. It's like asking what the "best survival knife" is. There are many great firearms, which will perform many tasks better than others. Even with that knowledge and the knowledge of a specific scenario, I wouldn't want one. I want a few in a designated cache.
Disclaimer: Obviously this stuff involves a lot of broad generalization. Specifics would come into play.
Hanguns:
To me, handguns are only useful as a defensive weapon in an environment where other people are concerned. Other than the "urban" setting, I would always rather have a rifle or shotgun. People will argue that a handgun is lighter weight. True, but I could do with a couple more ounces of weight for something much more accurate. The truth is, I'm magnificent with a rifle or a shotgun. Handguns, I have good grouping and can hold my own in a combat situation, but I'm not going to be making any serious accurate shots with one from 50 yards away. Also people would argue that you can't exactly walk around on public trails and such with a rifle or shotgun. A handgun is concealable. True also, but if it's a SHTF situation, I don't care about breaking the law to a certain extent. I will admit that if you're having to defend or be an aggressor, a sidearm is a good thing to keep firing on a target should you happen to run out of ammo for your primary weapon and need to either acquire more, take cover to reload, or pick up an enemy weapon. However, if you're in a group, others can keep firing on a target while you fall back to a position where you can reload your mags and such. There's more to this concerning suppression fire, being specifically engaged as a target, etc., but I won't get into it. So, if it came down to it, you probably wouldn't find a handgun on me in a SHTF situation.
Surplus semi-auto rifles:
I'm a big fan. SKS's and AK's are my bread and butter. Ammo is inexpensive and easy to store. These rifles were made to be simple, accurate to a certain extent, reliable above all else, and sure to go bang. I'll be more addressing SKS's than AK's. These are accurate enough to acquire large game from a distance. They are easy to clean. There are a ton of pros. There are however a few cons. They do require practice and accuracy on the shooters part. If you're not a good shooter, the gun is useless. ammo for these weapons is heavy. Carrying a lot is not a good idea. It would be better to carry only as much as you need for your task and leave the rest in a cache. Re-up when you head out next time. The weapons themselves tend to have a good weight to them. If you don't need to carry one around with you, it's a good idea not to. For anyone who has ever walked around a gunshow with a "for sale" sign on their SKS for more than an hour, you know how heavy they get quick. Doesn't matter if you have them slung or what. When you start out, they weigh only a couple of pounds. After an hour, it weighs fifty pounds. That makes this rifle a good rifle to carry only for the tasks for which you need it. Any other time, leave it in the cache with the ammo.
Rimfire rifles:
GREAT! GREAT! GREAT! These weapons are available in many different sizes, even small enough to be strapped to the side of a pack without in significant impact on weight. Ammo is the least expensive ammo in the world for any firearm (unless you're in Africa. 7.62x39 seems to be free there.) and is the smallest and most easily stored. You can carry much more than your task requires to no ill effect. There are perfectly capable of getting small game and some medium size game including pigs. Now for some cons. Although you can still hit a target very easily with this weapon, it's not an effective battle weapon. It doesn't do sufficient damage quick enough. It's not intimidating when fired. bla bla bla. For a defensive or aggressive situation, a rimfire is better than nothing, but not by much. So, if your SHTF scenario means that you will have to defend, get something bigger. Now, even though this weapon doesn't weigh very much at all in most configurations, it would still be better left with your cache when not being used for a task.
Shotguns:
Most are large, ammo is large, and it's not an effective long range weapon for taking game. However, in a defensive situation, shotguns are hard to beat. Accuracy is not as big of an issue. That is also true with taking short range game. a 12 gauge shotgun will make anything easier. You have to be Annie Oakley to hit a book moving through the air 10 feet from you, but retards with 12 gauges can nail clays all day long from a good distance. I am one of those retards and trust me, moving target means shotgun. Stalking a target and waiting for it to hold still for a good shot = rifle. Moving targets = 12 gauge. Also, let's not forget, 12 gauge ammo is inexpensive as well. As with just about everything else though, other than taking it out for use for a certain task, it's best left in a cache.
Over and under shotgun/rifle combos:
Very cool. Just carry two types of ammo. For everything else, see above. The same characteristics apply.
Moral of the story is this: Have more than one. Keep them in a cache. Take one out at a time for whatever specific task you need to use it for. If you have to change cache locations, be glad you were smart enough to buy slings for them before the local economy and trade system collapsed, and carry all of them to the new cache sight. Then, go back on one-at-a-timin'. In a defensive situation, grab a surplus rifle or shotgun depending on how many targets you're defending against. Right before the "great collapse", sell all of your handguns but one to someone else and use the money for rifle and shotgun ammo.