Weapon calibers are very often specified in fractional values. From an everyday point of view, it is not very convenient to write and pronounce such names. Of course, most people simply do not think about this and have long taken the pronunciation of fractional gauges into a habit. However, one question still remains: why shouldn't the gunsmiths and the military do a good deed and start rounding up?
In fact, this practice has a very specific reason, which is simple to the point of banality. However, first, it is worth talking a little about the fact that weapon calibers do not round off. In fact, they round up and how! And sometimes they even distort them for various reasons. Moreover, all this is equally true both for the British-American inch caliber designation system and for the European metric caliber system.
Just an example: a 9x19 mm Parabellum cartridge. Its real caliber is 9.01mm. However, for convenience sake, the actual caliber is rounded off from the moment it appears. In the case of a rifled weapon, it is also important which system (along the grooves or along the bottom) the caliber is measured. Another example: cartridge 9x19 mm PM - caliber can be designated not as 9, but as 9.27 mm. Rounding again! Another example: the already mentioned 7.62x39 mm - the real caliber of such ammunition is 7.92 mm.
And specifically with the 7.62 marking, there is a whole story. Initially, this caliber was measured with "lines" (hence the name of the rifle "three-line"). However, already in Soviet times, lines and inches were abandoned in favor of millimeters, after which the three-line cartridge became the 7.62 mm cartridge. Finally, we can recall the American 5.56 mm, the real caliber of which is 5.7 mm. Thus, we see that there are both roundings and "distortions" in the labeling business.
But why not round all the calibers to some integer values? First of all, because the calibers of small arms are too small relative to the calibers, for example, of artillery. Therefore, there are a huge number of them, especially if you count with experimental calibers. In view of the terrible variety, the question arises of the need for a clear illustration of the differences between the available calibers. Moreover, modern fractional calibers were originally developed as a result of the transition from the line system to the notation system in millimeters. So this is also just a tradition, which cannot be gotten rid of, including for bureaucratic reasons. Just imagine what an array of documents in each state is, which contains fractional designations of weapon calibers.
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In the end, the question of marking this or that ammunition is the business of the designers. Small roundings in designations are harmless. But rounding off the name of a cartridge by a few tenths at once can cause confusion and cause a lot of harm.
If you want to know even more interesting things, then you should read about why in the USSR an automatic rifle appeared in 1949, and in the USA only in 1964.
A source: https://novate.ru/blogs/020721/59615/
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