The "common caliber" ammunition strategy - have all your guns in a limited number of readily available calibers--seems to, in many ways, fly in the face of good ol' diversification. If all your handguns are in 9mm and you run out of and can't replace 9mm, then all of your handguns are now expensive paperweights. Good job.
Lots of people have been running into this problem as of late, with .22lr, .223, 9mm and other common ammo choices disappearing from shelves. Their guns are in those popular calibers, and now they're unable to find any ammo for their guns.
When does it sticking with common calibers make sense? When might it be a bad idea?
Ammunition Supply Problem
I often talk about what I call Ragnar's Rule of Three's, from 80s survival guru Ragnar Benson, who extolled the wisdom of having three ways to accomplish any critical tasks. The task we're concerned with here is supplying ammunition for our firearms.
So, let's list out ways that you can solve this problem:
- Have a large stockpile set aside in case of troubles
- Purchase/barter for more ammunition
- Reload your own - have the tools, components and skill
- Scavenge more ammunition (not viable in many scenarios)
If you've got three in place, then you can have all the advantages of ammunition commonality without worrying about running out of ammo. With two in place, I probably wouldn't lose sleep over it, either - a large stockpile goes a long way.
On the other hand, if you only have one method of feeding your firearms, you may find yourself exposed to troubles frequently.
The Average Gun Owner
The average gun owner doesn't reload, doesn't have a large stockpile on hand. He swings by Wal-Mart or the gun shop and picks up a couple boxes of cheap stuff on his way to the range. Scavenging (or looting) is not really an option--things are a bit crazy, but we're not at full on Mad Max by a long shot. With purchasing very difficult/expensive at the moment, the average gun owner is going to have trouble feeding his firearms.
He has one method for supplying his ammo (buying it), and if that doesn't work, he's out of luck. Having all of his guns in a limited caliber selection only makes that problem more likely and more severe when it happens. Diversified he is not.
As you can probably see, when you fall into that group of folks who can't/doesn't reload or have a large stockpile of ammunition, the whole common caliber strategy starts to make less sense. You've made yourself somewhat reliant on an outside source for ammo, and any interruptions to that source of ammo will leave you screwed.
If that's the position you've put yourself in, why would you pick ammunition that's most commonly hard to get a hold of? Why would you pick the same stuff that everyone wants and that is first to disappear from shelves during a panic? And why would you have all your guns run on the same ammo?
If you're dependent on outside supply for stocking ammo, it would be in your best interest to be able to use the widest variety of ammunition possible - not just one particular caliber, and especially not one particular caliber that is prone to panic buying.
Real world example:
Despite the shortages, SOME ammo is still in stock. Quick look shows .45 ACP, .357 SIG, 7.62x54R and .30-06 in stock at Palmetto State. All very good cartridges capable of shooting bad guys dead and putting meat on the table. Of course, if you had three pistols in 9mm and two .308 rifles, that wouldn't do you a lot of good, would it? But if you had two 9s and a .45 and a .308 and a .30-06, then you'd be in better shape, right?
Now there are options out there that allow you to fire a variety of cartridges from the same firearm.
For their many downsides, a break-action shotgun and a small collection of chamber adapters would provide a survivor with an omnivorous firearm at a manageable cost. A break action shottie and under $200 in chamber adapters would give you the ability to shoot 9mm, .38 special, .44 magnum, .45 Colt, .22lr, .22mag, .410 and 20 gauge shotgun. Your overall effectiveness would be moderate, but things would really have to be grim if you could not buy/barter for any of that wide variety of ammunition.
Similarly, break action rifles like the H&R Handi-Rifle and the T/C Contender allow you to swap a new barrel onto your action and start shooting an entirely different caliber.
Similarly, there are rifle adapters that work fairly well in bolt action guns. The 30-06 to 32 ACP adapter is pretty common - here are three options from Sportsman's Guide for under $20 each. Other options are out there.
Some Glocks have a limited ability to switch between calibers. A .40 S&W Glock can, if I remember right, be converted to .357 Sig and 9mm with little more than a barrel swap and different mags.
The AR-15 can be converted to a variety of different calibers by swapping out the upper - most popular semi-auto handgun cartridges are an option, as is .22lr.
If you look hard enough, you might be able to find a Medusa revolver, a long out of production revolver capable of shooting wide variety of different cartridges in the .353 to .357 range - 9x18 Mak, 9mm, .380, .38 Special, .357 Magnum and more. Exotic, probably not very practical but very cool.
Less exotically, if you've often lusted after a particular firearm, but have dismissed it because it doesn't fit with your "core" common calibers, I'd take a second look at it. Especially if your core calibers are prone to panic buying frenzies and new gun is no. The slightly less popular cartridges seem to have had quite a bit better availability - .45 ACP, .357 Sig, 10mm, and even .40 S&W have all generally been easier to find in quantity than 9mm. Deer rifle and revolver cartridges, too.
Likewise, if you've got a gun or two or twelve that don't seem to "fit" with your core ammunition choices, I probably wouldn't sweat it too much, as long as you're not hurting for cash or ignoring other areas.
It's probably pretty obvious that having a stockpile of ammunition is important, as is the ability to reload your own. That's the smartest way to ensure your ammunition supply, without really needing to get into a large collection of firearms or odd-ball multi-caliber options.
If you aren't or can't do either of those, you'd might as well be able to shoot the broadest array of ammunition possible, since your consistent supply of one kind of ammunition is uncertain. It might take a bit of creativity, but it certainly is possible.