Why is the mandatory caliber: 5.56 7.62 12.7 mm or something?

Because foreign countries, especially the United States, say that the caliber of guns is not millimeters.

For example, they say that the 50 caliber is 12.7 mm,

When we divide 12.7 by 50, we will know how many millimeters the American caliber is.

12.7 divided by 50 and multiplied by 30 is the American caliber of 30. We were surprised to find that the American caliber of 30 is 7.62! ! !

It turns out that these strange numbers of rifles are all American! ! !

30 caliber is 7.62 50 caliber is 12.7.

Then you can figure out what 40 caliber and 45 caliber are! ! !