Whether you own a firearm or not, you should be buying ammunition

Obviously, if you own a firearm, you should possess a sufficient amount of ammunition for it. Defense ammo (hollow point) is great, but range ammo makes holes, too.

But when the shit hits the fan, you will also be able to trade ammunition for food, water, fuel. It'll be a more reliable medium of exchange than currency, and it doesn't take up a lot of space. This makes it worth stocking up on for anyone.

I don't pretend to be an expert, but there are definitely callibers that are more common than others:

• 9mm (9x19, 9mm Luger, 9mm Parabellum) These are pistol rounds, very common for use in semi-automatics.
• .22LR Standard 22 caliber round, pretty much everything that fires a 22 (pistol or rifle) will use these. Very popular, very inexpensive.
• Other pistol rounds: 45 ACP, 40 S&W, 380 ACP, 38 Special While these are going to be less widely sought after, people who have guns that use these will need them.
• .223/5.56 These are rifle rounds, very common in AR-style rifles. Of note, the 5.56 round contains more powder and a heavier bullet. In most cases, you can shoot .223 in any rifle chambered for .223 or 5.56, but shooting 5.56 in a .223 rifle can damage the gun (possibly catastrophically).
• 308 Winchester Also a rifle round, primarily used for large game hunting (think deer, bear, maybe moose). If you're in an area with a lot of large game and their hunters, this round may be valuable
• 7.62x39 AK rifle round, also used in some AR. This is a light military round, so it won't have as large a market as the others, but has the potential to be very valuable to the right person.
• 12ga 12 gauge shotgun shell, as suggested by @shittydwarf@sh.itjust.works. Buckshot, since slugs can be damaging to shotguns that are tightly choked for shot.
• Other shotgun shells 20ga, 16ga are used in smaller shotguns.

Don't broadcast far and wide that you have ammunition. You risk making yourself a target. Just like a wallet full of cash, don't go flashing it around.