In a Dead Tree magazine (ergo no link, sorry), I remember reading that Tami Keel talked about ammo shortages and one’s preparation for them, and I’ll talk about that in another post after I find the mag.
She also addressed the issue in an earlier piece in Shooting Times (talking about the Great Covid Ammo Panic) and noted:
Retail stocks of common handgun calibers, such as .380 ACP, 9 mm, .40 S&W and .45 ACP, were quickly depleted.
Which they were. However, she went on to discuss other, less popular calibers, and noted that stocks of stuff like 9x18mm Makorov were less affected — and swapping out the .380 barrel in a Glock 42 for a different one in said chambering meant that one might be less affected by ammo shortages.
I myself saw that when I went to the local Scheels store during the Great Covidiocy, among the otherwise-bare shelves were plentiful supplies of esoteric calibers like .460 S&W, which makes me ponder the question (as Tami did):
Is it worth getting a gun chambered in an offbeat caliber as a backup for one’s regular (for example) 9mmP or .45 ACP?
It’s an interesting thought. Ordinarily, I myself would not be in the market for a gun chambered in, say, .44-40; but faced with a shortage of my regular ammo, that venerable cowboy boolet would sure as hell make a decent backup — especially if one had a rifle chambered for the same cartridge.
So let me look at a couple of “companion” options.
I’ve written before about budget-gunmaker Rossi’s R92 lever rifle, but specifically about its desirability when chambered in .357 Mag — one of those calibers that were severely affected by the Covidiocy shortages.

However, the R92 can also be had in .454 Casull (which can also handle the .45 Colt easily), and when paired with a Magnum Research BFR:

…that’s a hell of a combination. Manly, even. (And .45 Colt is another cartridge that may not be too vulnerable in a supply shortage, but maybe more so than the .454 Casull.)
How about the .350 Legend? (Also known as the .45-70 Govt’s little brother.) I also note that Hornady makes a subsonic variant…

Here we have two outstanding choices — the S&W Model 350:

…which can be coupled with (say) a semi-auto Ruger AR556 MPR rifle:

…or the bolt-action Howa Mini-Action:

All three look quite toothsome, don’t they?
Remember, in both the above two calibers, we’re not talking about an “everyday” shooting gun: we’re talking about having a gun, or pair of guns, that will do duty in a pinch and which will probably not fall foul of ammo supply shortages during a period of panic. (That the .350 Legend is not a bad choice for close-range hunting is a feature and not a flaw.)
Feel free to add other such cartridge choices in Comments. I’ve talked about a rifle / handgun pairing, but that’s not a prerequisite for this exercise. You can go with handgun-only or long gun-only.
Afterthought: I know, I know; one should always have enough ammo to withstand a temporary shortage — exactly how much, I’ll cover in that follow-up post — but hey, it’s a nice exercise anyway.