Ammo will be in short supply during any grid-down situation, and the only rounds you’re likely to have are the boxes in your storage (which is why you should stock up more than adequately).
Even so, you may still be able to procure ammo via trade with other people, and in those cases, the ammo that was the most popular before the grid went down will be the easiest to find and trade.
Though no one can predict the future, it’s likely that these will be the most common rounds in a grid-down situation:
.22LR (also handgun): Without a doubt, this will be one of the most valuable calibers in a grid-down situation. With its popularity, low recoil, quietness and versatility, the .22 should be one of your top priorities when it comes to stocking up on ammo.
.223 Remington/5.56x45mm NATO: This took up the biggest market share for rifle ammunition last year due to AR-15s being the best-selling rifles in America. If that’s not enough, this is a current military round.
.30-30 Winchester: A lot of Americans today still prefer the traditional lever guns, so there’s no doubt that the classic .30-30 will be in high demand.
7.62x39mm: AK-47s are both feared and respected, and along with it, the 7.62x39mm. With the ban on certain imports from Russia, it’s unclear about the future of this round, but for now American manufacturers seem to be producing more than enough of it. Even if you don’t have an AK yourself, other people do, making this ammo a valuable commodity for trade.
.270 Winchester: Quite a few American shooters swear by the venerable .270 to this day, and for good reason.
.308 Winchester/7.62x51mm NATO: The .308 has overtaken the .30-06 as the most popular big-game hunting caliber in the lower 48, but the margin is still razor thin. Also, as a current military round, the .308 will be one of the most important calibers circulating.
7.62x54mm: Chambered for such Soviet weapons as the Mosin Nagant, a very cheap and popular surplus rifle.
.30-06 Springfield: The enduring “thirty aught-six” will remain as traditional as steak and apple pie. It may have since been phased out by the military, but that hasn’t diminished the .30-06’s popularity with everyday people. The caliber is easily available and at fair pricing.
.300 Winchester: An excellent combination of high power and high velocity, the .300 Winchester has proven itself to be worth considerable respect. Many hunters have turned to the caliber for this reason.