Limits of 5.56mm
Now of course, there is merit to the battle rifle argument. 5.56 bullets are lighter weight and generally rely on fragmentation to do much of their damage. In order to fragment reliably, the bullet needs to be going fast when it hits the target. 5.56 can't maintain the speed needed to frag over long ranges, so hits at range are less damaging...basically an up sized .22lr. With M-193 out of a 16-inch barrel, you've got about 150-160 yards where the round will reliably fragment and you'll get your best tissue damage. Out to about 175 you will still get some good fragmentation/yawing. Outside of that, it's a 55 grain FMJ going fast. You can read more at the AR15.com ammo oracle.
So that's the general performance of the 5.56 round--under 200 yards. If you are looking to go out a bit further, there are alternative/better bullet designs and those designed for longer distance shooting; the 75 and 77 grain rounds like the 77 grain Black Hills SMK will yaw and fragment out to as much as 250-300 yards out of a 16-inch barrel. But in general, you're looking at 200 yards and under for optimal wounding capability.
Further than that, you've got a speedy ice pick. Now we know that NO ONE wants to get shot, and we also know that shot placement trumps everything else. An ice pick through the eye socket is a pretty definitive fight stopper. Similarly, a fast moving 55-77 grain bullet to the CNS will drop a baddie cold, whether it's at 50 yards or 500 yards. The same bullet to the heart, femoral, etc. will similarly ruin a threat's day. But yes, at extended ranges, the 5.56mm doesn't have a lot of oomph to it.
Finally, lighter weight, easier-to-fragment 5.56mm bullets do indeed lack the penetration capabilities of .308. The lighter bullets deflect more easily and are more likely to fragment than bigger bullets.
Trade Offs - What's more important to you?
Unfortunately, and despite what some would have you believe, there are no perfect weapons. There's no general purpose, do-all rifle. If you have to pick only one rifle and caliber, you're going to make some trade offs. We've looked at the downsides of 5.56mm. Let's look at the commonly discussed benefits of upsizing to .308.
Range - .308 is effective over much longer ranges than 5.56
Penetration - .308 has better barrier penetration than 5.56
Stopping Power/Big Game - These are two sides of the same coin. .308 has more energy behind it than 5.56.
I'm not going to argue those points--that would be silly. .308 is a bigger, heavier round with more powder behind it. It's bigger, goes faster and keeps its speed/trajectory better. What I will point out are the trade offs that .308 brings with it:
Added Recoil/Harder to Control: All that extra energy means that a .308 will have more recoil than a 5.56. Maybe not a big deal for single shots from the bench on the range, but things get interesting when you start firing off controlled bursts. .308s are much harder to control during rapid fire.
Additional Risk of Over-Penetration: .308 keeps going and going. Of course, you should always be mindful of your target and what's beyond, but can be a liability. SWAT teams generally favor the 5.56mm because it is less likely to over penetrate/more likely to frag when it hits a wall, concrete, etc.
Reduced Magazine Capacity: Bigger cartridges, reduced mag capacity. Most .308 semi autos have 20 rounders.
Long Range Optics: If you're going to take advantage of the longer range accuracy offered by the .308, you'll need a long range optic--magnification. These kinds of optics can be slow and clumsy when used for closer ranged work, where unmagnified red dots excel.
Added Weight: Bigger rifle, bigger optic, bigger cartridge. Heavier load out.
Additional Cost of Ammunition: .308 is more spendy than 5.56mm, and rifles tend to be pricier too.
5.56mm is more practical for most survivors.
Think through your likely use scenarios. Home defense. SHTF defense of your home/property. Running neighborhood patrols, watches and checkpoints. Bugging out by vehicle. Bugging out by foot. Consider the benefits and tradeoffs, and a 5.56 (or similar) rifle, set up for close to mid-range, comes out ahead for most of survivors.
Not all of us - most.
The .308 certainly does some things better, but again, think through your likely use scenarios. Your "general purpose rifle" should work for the general purposes that you will need it for. Most of my likely use scenarios don't include 400+ yard shots or need the penetrating power of .308.
The REAL Answer.
The real answer is, of course, to get both. While most of my likely use scenarios favor 5.56, that doesn't mean that I can't come up with a couple dozen possibilities where I'd rather have a .308. Guns are tools and there are purposes for weapons in each chambering. In a long range, sniper-type role, a .308 rifle is the clear choice. In the open desert, post complete-TEOTWAWKI, where the rules of engagement are to snipe anything threatening looking from maximum range, I'd go with a .308. Hunting mutant zombie bears - .308.
As I mentioned earlier, there is NO perfect rifle. No one weapon that does it all. If you want to do it all, you'll need several different weapons. If you can't afford (or carry or feed) several different weapons, look at your most likely use scenarios and pick the one that works the best for the highest number of those.
