There is something to be said for ammo commonality, if you're part of a group, and especially if you're the guy in the group who forgot / lost his ammo.
But, if you aren't that guy, then, not so much. This lesson was taught to me early in my deer hunting days with my father, uncle, and some of our other relatives/buddies who hunted together. We hunted in the Adirondacks, mostly "drive hunting", with drivers and watchers. Get up 0 Dark Thirty, pack up, head out so we could be getting on the mountain about first light.
One time, as we were unloading our rifles and gear getting ready to cross the field to the woods, it turned out that Hiram had forgotten his ammo. So, he's trying to bum shells from the rest of us. He had a .30-30.
I couldn't help, I had a .308. Art had a .30 Rem. Lester had a Marlin .35. And Dad had a Winchester .32Spl. BUT, the other three guys with us all had .30-30s, and each gave up a few shells so Hiram had a full load for his rifle and enough for a reload. And we went on with the hunt. In this case, ammo "commonality" was serendipity, but for Hiram that day, it meant the difference between hunting with us or having to go back home, or to town and wait for the store to open, for ammo.
Another friend told me about a place he remembered from growing up, a Mom & Pop gas station/diner/general store in a little town "on the edge of the wilderness". During hunting season, the place would open at 2-3AM catching the hunter traffic on their way to the woods. Gas, coffee, breakfast maybe (and of course, a bathroom,
).
What made this place memorable was that there was a long set of shelves with at least one box, or partial box of about every kind of ammo known to man. And the owner would sell them by the round! And, on top of that, if you didn't shoot them, he would take them back, minus a small fee. Some of that ammo was decades old, but when you are 4 or 5 hours drive away from a store (which can only really be counted on for common stuff), and you find you've forgotten your ammo, a place like that is a miracle. and a double miracle when it actually has the "oddball" round you shoot.
Yeah, he gouged, $3-4 a round (in the 70s!), but when the only other place you know for sure has your ammo is 150 miles away, and you need it, its pay up, or go home. And, like I said, if you brought it back, he'd take it back, and give you back most of your money.
Nobody does business like that these days, so it seems. "Stupid" business model, to modern thinking, having all that money tied up in inventory that rarely sells. But, I think he knew what he was doing, Save one guy's hunting trip, and you've got a customer (and likely a friend) for life.
Every time that guy comes through town, if he needs anything, he going to stop in your place, maybe just for a cup of coffee and to shoot the breeze, but he's going to bring some business. And, he's going to tell his friends, too.
On a different tack, still on the subject of "common" calibers, one often hears the advice to stick to A or B so that in times of disaster (or TEOTWAWKI - which we don't do here at TFL) you can get ammo "resupply".
The usual argument involves "finding" common ammo "in the ruins" or taking it off the dead bad guys who tried to kill you, or joining up with others who use the same calibers.
I've never considered these as very realistic. Realistic is, the only ammo supply you can count on is what you ALREADY have.
Finding ammo resupply in "the ruins"
Long shot. Not impossible, but not likely,#1) it's called
looting, and you could be shot!
(and by "good guys")
#2) unless you are the first guy there, any common calibers are the first ones taken...that box of 7x61 Von Hofe Express in the abandoned shop or home doesn't do much to feed your AR or AK, or ….
I'll leave out taking ammo off the bad guys...not something for discussion here, its something for bad post apocalypse fiction novels.
Join a group? Ok, maybe, but expect that if you're bringing more to the party than they are, your stuff will be taken and "shared" with the group.
In short, if you're sticking to only the most common rounds, thinking you'll be able to get ammo when normal channels are dried up, Don't. Keeping it real, when there is a disaster, when martial law is declared, the first things done are to suspend LEGAL sale of firearms, ammo, and alcohol.
All you can really count on (until normal business returns) is what you already have. And since that's the case, if you have an uncommon round, and your own supply already, its not an issue, is it??
The niche rounds survive, because they do something a little bit better than the more usual stuff. Economy isn't usually it.