The gun manufactures are the smart ones, they use mm, gauge and caliber so everyone around the world knows the sizes they want to purchase or need.
Two out of your three examples have nothing to do with the Metric system. Gauge, as in shotguns, is defined as the number of lead balls the same diameter as the bore it takes to make a pound. Not a kilogram, a pound. A 12 gauge fowling piece requires 12 of the aforementioned lead balls to make a pound. Hence the lower the gauge the larger the bore.
Caliber is always expressed as a decimal fraction of an inch. Not millimeters, not centimeters, an inch. A 30 caliber gun has a bore equal to .30 inches.
Referring to your previous communique, a base 12 system would progress 12, 144, 1728... Not base 10 starting from 12 as you described. If the human race had evolved with twelve fingers it's likely that we would all be doing base 12 arithmetic instead of base 10. It's actually superior since 12 has more factors than 10.
Moreover, all atoms are not the same size. If you have a system that's defined by the diameter of an atom, it depends very much on which atom you choose. It seems suspiciously convenient that the atom the was chosen as the basis for the metric system just happened to fit the prejudices of the definers. What if another culture did the same thing but picked another atom of a different diameter.
Unfortunately a meter is defined to be the distance light travels in 1 / 299,792,458 seconds, not the diameter of an atom of any element. Worse as second is defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. No integer powers of 10 to be found.
These definitions seem to have been selected to fit what already existed. For years a meter was defined by a physical artifact. Arbitrary, as I said.
