Left to Right: .22lr, 9mm, .38sp, .308
I was doing really good for a long time. When I got into survivalism and prepping I did a lot of research and thinking and planning... I decided to get a pistol and a rifle that could share the same ammunition, a common round, reliable and easy to work on.
Eventually I decided on the Ruger Blackhawk Convertible, a revolver that shoots .38sp/.357 and can convert to 9mm, and a Rossi R92 lever action rifle in .38sp/.357. (Note: I haven't been able to do a review of the Rossi because it broke the first day at the range, there will be a full post on that later on). I figured that if things ever got so bad that I should get out to the woods for a while I'd have a nice rifle and pistol that shared and fired a good variety of common/cheap ammunition.
However, by the time I had my "survival gun" set up I'd already been bitten by the shooting bug. Once you have the basics it's hard to stop there. What if I want to go hunt bigger game or at a larger range? I should probably get something a little heavier. Or when you stumble over a super-cool gun at the store that would be a blast at the range (Henry Golden Boy .22lr).
In recent times I've started to more closely examine the differences between calibers. You can see the drastic difference between the ones that I shoot in that first photo.
.22lr is a great round for the range, low recoil, nice accuracy at shorter distances, all around fun. It's amazingly cheap, light, and you can put a thousand rounds in a pack without crippling yourself. It doesn't hit very hard compared to most other rounds, but you could probably shoot 10 for the cost/weight of most other rounds. Great in a short range rifle and is available in a lot of great pistols.
9mm is a great round for hand guns. It's cheap, light, extremely common, and packs a decent punch. Arguments have been made against 9mm stopping power but honestly, I think it comes down to what you're comfortable shooting. If you can shoot really well with a .40 or .45 than great, but for those who want lower recoil, faster handling, and usually a higher capacity, than 9mm is hardly a slouch. As for stopping power, there are studies that show that usually a torso shot will stop a person pretty quickly, caliber doesn't seem to be as much of an issue as where it's placed. Thus, shoot what you're comfortable with. There are some 9mm rifles as well, good to pair with a 9mm pistol, but there aren't many options and they are somewhat uncommon. The Thureon Defense rifle, Hi-Point, and AR-15 style Colt model 6450 in 9mm are some options.
.38sp/.357, I group these together because there is not a lot of difference between the two. The .357 is a slightly longer version of the .38sp with a few more grains of powder behind it. Almost all guns that can shoot .357 will also shoot .38sp. It's got a little more kick than 9mm, it's common in revolvers and was THE round in police revolvers for decades. It's slightly heavier, slightly more expensive than 9mm. What I like about this round is the variety, between the different .38sp and .357 rounds it's pretty easy to find the round you're comfortable shooting. The other benefit is how easy it is to find a good lever action rifle that shares the ammo. Some rifle nerds poo-poo lever actions but they have been historically proven and remain a good general purpose rifle.
.308 is by far the largest round that I shoot. This is a big round, kicks hard, and hits harder. It's the most expensive round, the heaviest round, and also the most accurate at longer ranges. The .308 is commonly used in long distance matches. It's also used in most military sniper rifles. When you just need to reach out and touch something... .308 is a good round to have available. This is basically just a rifle round though, you'd be hard placed to find a pistol to use the same round (and probably wouldn't want to shoot it if you did), which would mean packing at least two different rounds with you. Also, this isn't that much fun at the range, you put more than twenty rounds downrange and you're shoulder will be pissed for the next couple days. Good for one-shot one-kill, but follow ups will be more painful than with other rounds.
One of the things that surprised me when I was shooting more often was the different effects of the different rounds on wood. The targets I shoot are wood frames with a cardboard center, paper target stapled onto the cardboard. 9mm and .38sp when they missed the target and hit the wooden frame would have a small entry, big splintering exit. The low speed entry meant that the bullets had time to expand in the wood before exiting. When I hit the wood with .308 I thought it would be destroyed, blow the frame in half. Yet the opposite was true. Small entry, small exit, as though the bullet didn't even notice it had hit anything. It just went straight through it. Straight through two inches of wood at 100 yards? That's penetration.
That's something to keep in mind when hunting, the expansion of the bullet when it hits something solid. For short range hunting (or defense) a 9mm or .38 will work, it will hit, expand, and stop. On a small target at shoerter ranges that should be enough, something bigger and it might expand too fast, not penetrate far enough. For a larger animal .308 will pass through far more tissue before starting to expand. With most animals I wouldn't be surprised if the .308 went straight through. For larger animals that could easily be the difference between a wound and a kill shot.
The other thing to keep in mind is that pistol calibers in a longer barrel (like a rifle) will perform way better than out of a pistol. The longer barrel gives a longer area for the explosive gasses to expand before the bullet leaves the barrel, that gives the bullet a lot more velocity and range. A .357 in a pistol is a powerful pistol round, put it in a longer barrel and it starts to rival a rifle round for velocity and effectiveness. Yet, the rifle recoil in a pistol caliber is significantly reduced from a rifle round (they're designed to be shot from a hand gun, there's more weight in a rifle, less felt recoil). The .38sp in a lever is only slightly more recoil than a .22lr but the bullet hits a lot harder. They won't be a match for .308 in accuracy or distance, but it gives some credence to sharing a round between a pistol and a long gun.
There are articles, graphs, and websites that will back up the information here and others that will totally disagree. Feel free to peruse them and see if you can wade through the all various view points. The information here is based on my own experiences and you might find that your's varies. There is no substitute for personal experience and practice, so experience and practice as much as you can to find out what works best for you.
Hopefully some of these thoughts help you in choosing your calibers.