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Pistols tend to use larger caliber and heavier bullets than their rifle contemporaries. For example, .45 ACP bullets are much heavier than .30-06 Springfield bullets, and 9 mm Parabellum bullets are much heavier than 5.56 mm NATO bullets. The 5.7 mm pistol cartridge is somewhat unique in using a same caliber but lighter bullet than its rifle contemporary, allowing it to be more easily produced at facilities manufacturing 5.56 mm rifle ammunition (and this seems to have been the case with the .30-18 Auto cartridge designed for the M1903 Springfield Pedersen device modification), but obviously that approach isn't very commonly used.

Given the fact that it is much easier to modify something's length and that it requires a lot less retooling than changing diameter, why didn't a military or a company decide to produce ammunition to a standard caliber with differing bullets or case lengths?

Also, why do pistol cartridges tend to be short and fat anyways? Is it due to the ballistics involved, or is it a matter of it being easier to design a firearm around rounds that are short and fat instead of long and skinny (just like with naval ballistic missiles)?
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