During a crisis, you may not have enough food, or you may be beset by wild animals or people who are desperate or just have nothing to restrain their innate violence. Or critters may stop by to help themselves to your crops or livestock. A gun can be of help in these situations, and in order to be useful, you need to have ammunition for it.
For any particular use, there is one or more “calibers” of ammunition which is best for that. Stocking up on 20 different calibers would be impractical, particularly since you don’t know which use will be most prevalent.
We want the minimum number of calibers which will suffice for most likely uses, and we want calibers which are common and reasonably priced today, and are most likely to be available during a crisis. And we want the calibers to be desirable enough that if we have to , we can trade some for something we need desperately.
Check out my article on this: