Call me paranoid, but if .22 rimfire ammo is unavailable then probably half of the rifles and pistols in the US are without ammo. Out of just about any such rifle with open sights, a .22 LR can pretty reliably and decisively hit a man at 150 yards or more.
On the positive side, owners of .22 LR firearms have spent the last 5 years stocking up on .22 ammunition.