Not all, but still a representative part of assault rifles (automatic machines) has precisely curved magazines. A prominent representative of this family is the Kalashnikov assault rifle. The banana shape for the ammunition element was chosen by the designers for a reason. It has a deep technical meaning.
Any unitary ammunition, at least intermediate, at least large-caliber, in the vast majority of cases has a shape formed from a combination of several truncated cones. Moreover, if we average the shape of a rifle cartridge, then we just get a regular cone: with a dot on one side and an ever-increasing radius as we move to the opposite end.
And this is where geometry comes into play. How should the cones be folded vertically to be as compact as possible? That's right: fold the cones close to each other. If you lay out the figures in parallel, then they will touch only separate points at their base, which means they will take up much more space in space. This is the first and, in fact, the main reason why most rifle magazines are made curved.
There is another important point, because of which the banana shape is so popular among weapon designers. The second purely technical moment arises from the problem of the tightness of conical cartridges to each other. The fact is that if the ammunition is in the store not close to each other, but in parallel, then the risk of jamming when feeding a cartridge during firing increases. It is possible to solve this problem by other technical means, however, such an approach would be an excessive complication of the design.
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Of course, there are rifle magazines with a rectangular shape. They are used when the weapon fires cartridges, the shape of which tends not to a cone, but to a cylinder. At the same time, rectangular magazines can be used quite safely when working with rifle or pistol cartridges, since the sleeves of both have the shape of an “absolute” cylinder.
If you want to know even more interesting things, then you should read about obsolete weapons for non-professionals: myths about the Kalashnikov assault rifle.
A source: https://novate.ru/blogs/031121/61133/