| I was once called an expert in external ballistics, this may have been an overstatement but I did spend quite a lot of time writing simulations for the ballistics of various bullets. There are three phases that are important for bullets: what happens inside the barrel, what happens outside the barrel, and what happens when it strikes the target. There are a lot of parameters effecting that characteristics of all three. (not just 2 dimensions of size) Shape, muzzle velocity, reliability, ease of manufacture, etc etc. are all quite important. There is also long history, a gun is a thing that can be around for decades; you can't so easily throw away the past and start over. There is also a strong consideration for the design process. Fulfilling a set of requirements can conflict with the desire for standardization. Also when standardization is an idea, using an existing standard rather than making a new one has been the pragmatic response. NATO though has done quite a lot in standardizing American rounds. There are many almost-equivalent NATO standard rounds with things like metric measurements instead of the more historical American measurements. But also, things are just complicated. When talking about an airplane you might as well say that length and wingspan might "go quite a long way" in describing it. I mean, in some sense sure, but a very long way from fully describing one. There's a book "American Rifle" which goes through quite a bit of the history of the development of guns in the US. |