Is there any rhyme or reason to ammunition sizes that I seem to be missing? The range of calibers sizes appear to me fairly arbitrary. We have .17, .22, .32, .38, .40, .45 etc. The metric sized ammunition at least hits the round numbers, but they also have 5.56mm, 7.62mm among others. How come we don’t have sizes in .15, .20, .25, .30, .35, etc? Is there some regular interval of foot pounds, or drop, or something that physics dictates these are the sizes required? I don’t see it on the ballistic charts. Is it decided by the tooling required to make the rounds? Or what?