Inches to MM is a multiple of .03937

So if we look at it mathematically instead of in the shooters vernacular,

5MM = .1968" (almost 20 cal)
5.56MM = .2188" (almost 22 cal)
6MM = .2362" (remember the old 6MM Lee was called a 236?)
6.5MM = .2559" (the 256 Rigby was their version of the 6.5X54 Mann /Scho)
7MM = .2755" (Close to the 270 which is .277)
6.8MM = .2677" (so where in the hell did the US Army come up with that name for the 6.8SPC?)
.284" is actually 7.213MM
.308" = 7.823 Not 7.62MM
8MM = .3148" (the old 8mm Mauser with the J bore was .318", which is close but not exact)
9mm =.3543" 7 ten-thousandths off.
9.3MM = .366" (Wow, one that is correct)
9.5MM = .374" (very close to the .375 bullets used in that bore size)
10MM = .3937" (again close but not .400 as we have been told)
11.5 = .4527" (wow, another one that's right)
13 MM = .5113" VERY close to the 50 BMG which is .512"